Demonic Spiders / Destiny 2

Even the mightiest of Guardians have trouble with these bastards.

Cabal enemies

• Scorpius Drones. They appear rarely, can't move (being Turrets) and have pretty low health, but can bring even a Player at full health using a Super to low health in just a few seconds of its constant fire even from a distance. It's recommended that you take these out first, as once they set their sights on you, you'll face a nonstop hail of bullets akin to a Colossus that just doesn't flinch. The only thing that can sometimes stop them is grenades, as they can go into a hunker-down state like some other enemies.
• Incendiors, especially on activities with high Power requirements. Because they deal damage over time, it's easy to forget you are staying still in their trail of fire, and the residual damage stacks up fairly quickly. Shooting their fuel tank also results in a violent explosion that's most likely to kill you if you're anywhere near them, and if you didn't instantly get blown up by that, residual damage will clean up whatever sliver of health you have left. Oh, and if you didn't vaporize the Incendior completely, its fuel tank will remain on the ground, potentially blowing up in your face if anything triggers it. Just to rub salt in the wound, even the lowliest Incendior can do a physics-breaking Shockwave Stomp whenever you approach them; the same damned Scrappy Mechanic reserved for bosses and larger enemies! Bungie clearly knows how much of a pain in the ass they are, as they're one of the enemy types that can be an Unstoppable Champion - which means they have massive health pool and can't be staggered without the appropriate anti-Champion mods. Ouch.
• Gladiators are basically War Beasts except they're bigger, have more health, do more damage and are slightly faster. They also tend to single you out of cover while you're recovering health. They can also One-Hit Kill you if you touch a wall while they're hitting you.
• Cabal Colossi. Fittingly for the strongest Cabal standard unit, they will decimate a Guardian's health with either an automatic cannon that hits like a truck with every shot (on top of angrily shaking your screen due to the incredible impact), or a gatling gun which whittles your health bar very quickly. If there's any kind of distance between you and them, they'll forgo using their gun and start sending endless barrages of missiles which slow down and blind you, meaning you're more likely to get hit by the next barrage which immediately launches after the first one until you break line of sight. If you get hit by any of the missiles, there's nothing short of a miracle stopping you from immediately getting hit by the next barrage.
• Threshers flew headlong into this territory in Lightfall. The main issue is their missile barrage, which is totally silent and can easily catch players off-guard in a firefight. Then, to compound the issue, Threshers have a huge amount of health, and no crit spot. And just to make things worse, Lightfall introduced a bug that affected several enemy types, including Threshers, causing them to do signifigantly more damage at higher framerates.

Fallen enemies

• Scorch Cannon Captains. Like the first game, they deal a ton of rapid explosive damage and have high health AND shields. Unlike the first game, they'll now start teleporting a lot around you when at low health, making it harder to land shots (and potentially wasting Power or Energy ammo). They can also fire their shots like a Mortar if you decide to use a Barrier (as a Titan) in front of it or get into cover right in front of him.
• The Berserkers from "Scourge of the Past" are modified Fallen Captains that are both invincible and invisible at a distance, requiring you to close in to break their shield. Once you get close, however, the Berserker will deploy a field that suppresses all of your abilities, leaving you with only your guns to destroy two weak spots that are located at opposite ends (this usually requires two Guardians to tag-team the Berserker), else you get oneshotted by the Berserker's beam weapon. As if it wasn't enough, breaking the shield will send the Berserker on a very aggravating teleportation spree which would make even regular Captains faint, and the suppression field sometimes has a nasty knockback effect that can effectively turn you into a victim of physics or shut you mid-flight and into a hole.

Hive enemies

• Wizards. They're one of the few enemies that perpetually float in the air, making abilities and weapons that work best on the ground (such as swords) nearly useless against them. Having a shield also makes them tough cookies in activities with the Match Game modifier. Not only can they keep blasting at you with bolts of lightning that deal deceptively high amounts of Splash Damage, but they can also create clouds of black smoke that cancels your jumping ability, continuously damages you and slows you down within it, leaving you vulnerable to other Hive threats like Cursed Thralls; given that Wizards are considered supporting enemies and thus are never alone, this will happen all the time unless you go for them first.
• At first glance, the Hive Knights' shield/regen ability isn't as bad as Hobgoblin stasis, since it can be flanked. The issue is that while they heal a similar percent of their health, it's Hobgoblins and Knights, so you end up wasting a lot more bullets on them. To make things worse, they seem to — unlike Hobgoblins — have virtually no cooldown on this ability, and yellow-health Knights will happily spam their shield and healing to their shitty little heart's content. As a result, the Revenant Knight turns into a Damage-Sponge Boss in Mook's Clothing that won't go down unless your whole fireteam is constantly pummeling him from multiple angles. And it's not exaggeration to say at least one appears in every major fight of the Savathun's Song Nightfall.
• Shriekers; while they no longer send out explosive homing projectiles once they're killed (this is now a special attack for rare Boss Shriekers), instead of firing in slow bursts they fire a neverending stream of Void projectiles once they target you. They're also escorted by many more Hive, and in the "Savathûn's Song" Strike they even have to be taken out by a energy orb that leaves the holder weaponless.
• The Lucent Moths introduced in The Witch Queen. Any type of Hive can now spawn with an elementally unaligned layer of shielding indicated by a blue bar on their health, even enemies that don't have any shields at all. If you break that shield, the Lucent Moth flits away and finds another Hive enemy to shield instead, so if you don't shoot them down they'll make every fight take about twice as long. Their flight is unpredictable, sometimes hovering in place and making an easy target, sometimes zigzagging like mad - and if there's no Hive hosts in range, they'll fly at you and explode like a grenade. On the bright side if you do pop them, they release that Arc explosion to damage everything nearby, potentially leading to a deliciously explosive chain reaction if there are other free-floating Moths or a few Cursed Thralls around.

Vex enemies

• Hobgoblins are as obnoxious as they were in the original game, typically spawning in faraway locations to snipe you and force you to switch to a long-range weapon. If you don't kill them in one shot or knock their head off, they'll immediately activate a shield that makes them invulnerable for a few seconds and regenerates their health; a moment's notice that's usually long enough to divert your attention away from them while they get back in position to snipe you.
• Hydras are completely overtuned in Gambit, dealing significantly higher damage with their Aeon Mauls than in any other PvE mode. This makes tackling them a very risky affair, even if they're the only enemy in the area.
• The Wyvern introduced in Beyond Light quickly gained notoriety for being even worse than Minotaurs, outstripping their mobility and durability while having the firepower of a Hydra. Their Warp Lance is nothing to scoff at despite their slow travel speed, as it doesn't take many bolts to connect to utterly ruin your health. Then there's their Aerial Slam, in which they perform a divebomb on your location and inflict knockback similar to a Taken Phalanx's shield blast. On Ordeal Nightfalls, the Wyverns gained a nasty reputation of being more dangerous than even the Champions due to their attacks becoming a death sentence on all accounts.

Taken enemies

• Taken Centurions. They don't appear too often, but they have a fairly tough Arc shield and a homing projectile that can seek you out even when you're behind cover. You can shoot it to destroy it, but that'll alert the enemy to where you are.
• Taken Captains, just like their D1 selves, constantly teleport around, making it hard to reliably hit them, and they launch a very large orb of darkness which obscures your vision for several seconds on top of dealing heavy damage, potentially killing you in one hit in endgame activities if you get nailed by the entirety of the orb's path. Elite versions can fling these shadow orbs constantly.
• Taken Knights are just as frustrating to fight against as their Hive counterpart listed above. Even though you don't have to deal with a shielding ability, their Void Boomers are scarily accurate by virtue of not being bogged down by gravity like the Hive Arc Boomer, and they constantly spew a stream of Solar fire that makes Cabal Incendiors green with envy, denying you what little space you have when fighting against them. In enclosed arenas like the main encounters of the Prophecy dungeon, you will learn to hate their seemingly godlike accuracy and unrelenting aggression.
• Taken Wizards are even worse than their Hive counterparts. Their Necromantic Gaze deals a deceptively high amount of damage despite it being a slow stream of homing projectiles, and they can spawn an infinite number of Shadow Thralls to distract you away from them. Not only that, but they tend to float away to safety in case you try to approach them with a sword or a shotgun, all while continuing to blast you with their Necromantic Gaze.

Scorn enemies

• Scorn Abominations are essentially reskinned Hive Ogres with a powerful Arc bolt attack that can snipe you very accurately and from an extreme distance, making fighting Hobgoblins a preferrable option. Fortunately, they're a one-trick pony; either they're sniping at you or smashing you up close, and that's all they can do, even their Elite versions.
• Scorn Chieftains are some of the most annoying foes you'll ever face due to not only firing off a plethora of projectiles to stave off your approach (chief among them being the Arc mortar that keeps erratically bouncing in all directions), but also deploying a totem that inflicts a nasty effect on you depending on their element; if it's Solar, you'll just get a static totem that produces fire that's easily sidestepped, but if it's Void or Arc, you'll really have to destroy the totem to avoid fighting invulnerable enemies and getting pulled towards the totem constantly, respectively. Thankfully, the totems are easily shot down, and the Chieftains are mostly relegated to Forsaken and Witch Queen-related content.

Champions

• The Barrier Champions introduced in Shadowkeep for endgame activities. They're much more infuriating to fight against than the two other Champion types since they can regenerate their health to full while their barrier is deployed, necessating you to keep firing anti-barrier rounds at them to slow down their healing. They also don't stop firing while deploying their shield, so you have switch to an anti-barrier weapon to shoot down the shield while they keep on attacking. On higher difficulties, having only one anti-barrier weapon may not even stun the Champion before the barrier retracts and they're already back to deploying another one. May the Traveler bless you if you happen to fight two Barrier Cabal Colossi next to each other.
• Overload Champions don't trail far behind Barrier Champions in terms of being frustrating to fight, especially while underleveled. A metric ton of health that's made even more aggravating by a Healing Factor that surpasses the Barrier Champion's own, all while the Overload Champion is completely Immune to Flinching until disrupted. And even then, the disruption doesn't last long before it's back to business attacking you, regenerating health be damned. There's also the fact that the seasonal mods that are meant to counter them are usually less effective than other Champion types and sometimes outright malfunction, leaving only Divinity as a reliable option.
• The Overload Captains are easily the most infuriating Overload Champions to fight against due to their Teleport Spam getting cranked up to eleven, making it hard to even land a hit with the weapons mods designed to counter them. Hope you enjoy Exodus Crash, because the Ordeal version features dozens of them!