With all the various enemy types in Terraria, it's no surprise that some of them will make you want to dunk them in lava out of sheer frustration.
- As you get better gear, previously challenging monsters go from threats to annoyances, but they still cause knockback even if they cause no appreciable damage. Even after you have a full set of Molten armor you can still be killed by a lowly slime slapping you into a pit of lava. That is, unless you have a Cobalt Shield or its upgrades to prevent knockback, in which case they go from a minor threat to practically nothing.
- Any enemy if you're trying to manage your items or craft something, especially worms. For that matter, worms in general, with their ability to travel through blocks and strike you from basically anywhere while being almost impossible to attack with early- and mid-game weapons until they're right on top of you. You will learn to hate the sound of a tunneling worm. Thankfully, the game gives you the option to auto-pause in your inventory menu, but you might not think to enable this until you've already gotten yourself killed while going through.
- Want to get your hands on some Meteorite? You'd better have the patience of a saint, since Meteor Heads will be constantly harassing you as you try to pick it up.
- The game has literal bats, and though they aren't that dangerous as long as you have good armor (save the Giant Bats in Hardmode that confuse you), they spawn constantly. On top of that, their hit-box is a single block, so ranged weapons have a hell of a time scoring a hit. And speaking of Hell, this is especially true in the Underworld, where up to five at a time may be harassing you, and down there they do enough damage that even Molten armor isn't enough to render them harmless. Considering you have to spend a not-insignificant amount of time down there trying to flatten the place out for the Wall of Flesh, this can get old fast. It's also possible you'll accidentally spawn the Wall of Flesh by shooting down an off-screen voodoo demon trying to hit one of the ever-present lava bats. In Expert, they're even more annoying—getting hit by them can give you the Feral Bite debuff, which increases your damage, lowers your regeneration... and also periodically gives random smaller debuffs on you, such as Confusion. It takes an entire minute to wear off.
- Spellcaster monsters have the ability to teleport to random locations near you and fire through walls. Though the projectile can be destroyed by a single hit from any weapon, they'll harass you until you chase them down and kill them, which only takes a few hits once you get there. This often means digging holes in every direction since they decided to teleport somewhere you can't reach. Fire Imps, the underworld variant, are even worse. The underworld is at least 75 percent lava at surface level, and all the good hellstone deposits require digging through the towers spaced between the lakes. This means the Imps often appear in out-of-reach places. They can and will show up when you're busy fighting Bone Serpents or mining. Oh, and unless you came with an Obsidian Skin potion or a Lava Charm/Waders, you have to deal with this on top of avoiding the lava and possibly not burning yourself on the ore. Have fun!
- Harpies. Flying enemies that appear at high altitudes and use a high damage projectile feather attack that shoots out multiple projectiles. Doubly so if you're trying to fish up there.
- Hoplites are one of the two exclusive enemies from the Marble biome, the other being the dreaded Medusa who is already listed as a Demonic Spider. That being said, Hoplites are still a huge pain to deal with, packing a wallop and a half with their thrown Javelins and surprising accuracy even at longer ranges. It's advised to bum-rush them as they cease their ranged attacks and "only" hurt you with contact damage. Failing that, you should either run or prepare to be eviscerated.
- Archers are annoying as heck. The hit you from a long way away, do hefty damage, and the Skeleton variant can set you on fire.
- Lava Slimes on Expert Mode. They’re no hard to kill and pose very little risk of actually killing you… until you kill them and the drop a block of lava that deals good damage. If you’re trying to build a large Hell bridge for the Wall of Flesh it’ll destroy platforms and campfires and if you use normal blocks you’ll still have to poke a hole in the bridge to stop the lava from damaging you. They also have an aggravatingly high spawn rate and can bounce off the lava pools if they spawn under the bridge or fall off it.
- Hellbats are just as annoying. They are not hard to kill and pose little threat but they can fly, spawn extremely frequently, and can blend into the background. The spawn rates in Hell are so high in general that killing one enemy often leads to 2-4 taking their places while doing so.
- The various Eaters in the Corruption, because they just never stop coming and spawn really quickly. The Crimson is better because its variant spawns less frequently.
- Gastropods shoot pink lasers from off screen with ridiculous speed and accuracy and deal massive damage for a common mob. Plus, they tend to gang up on you.
- Chaos Elementals run like crazy, charging you and teleporting all over the place.
- For a new player or someone setting out with a new character, Flying Fish and Demon Eyes are difficult to deal with due to their numbers and flying movement, making it hard to swat them down with melee weapons without taking damage and hard to hit with ranged ones.
- Icy Mermen have a pesky ranged attack that deals a moderate amount of damage and inflicts the Chilled debuff for a good 25 seconds, making you wait an agonizingly long time for it to wear off and making it harder to dodge subsequent projectiles. They can also rarely freeze you solid.
- Etherian Wyverns and Kobold Gliders in the Old One's Army event. They're the only Airborne Mooks in the event, whereas everything else is ground-bound, allowing them to bypass your defenses and go straight for the crystal to chip away at it over time. Additionally, none of the summons that the Tavernkeep sells can counter them reliably; this means that your towers will fire at them, miss, and let all the other enemies through, forcing you to constantly run around in order to kill them as quickly as humanly possible.
- Granite Elementals. They are pre-hardmode Airborne Mooks that can pass through blocks to get to you. In Expert Mode they're even more annoying, having twice the defense and the ability to turn invulnerable for a short time, meaning that it takes an extremely long time to bring them down with early weapons. They also tend to spawn well before you're within sight of a Granite Cave biome to pester you. Mercifully, only one can be spawned in a world at any time, meaning that it's impossible to be swarmed by them... but this usually means another one will come by if you destroy the previous.
- Desert Spirits in Corrupted/Crimson Underground Deserts in Hardmode aren’t difficult to kill or much of a threat but they have an insanely high spawn rate and are impossible to just ignore because they send projectiles that pierce walls. By the time you kill one, another one (or more) will immediately take their place.
- The Lunar Event is chock-full of these and Demonic Spiders. Most of them also have a great amount of health, requiring the strongest weapons to bring down.
- Nebula Pillar: Brain Sucklers appear in huge numbers and fly straight at you, even through walls. When they hit you, they latch onto your head and greatly obscure your vision and deal a good amount of continuous damage. Nebula Floaters fly around, fire painful projectiles, have a good amount of health and teleport when hit.
- Stardust Pillar: Star Cells are Asteroids Monsters that, when killed, produce four smaller cells which can each grow into a new Star Cell, creating a potentially endless swarm if you can't track down the cells or don't have a weapon that deals damage over time like fire. (However, since progression through the event depends on the number of enemies you kill, Star Cells can be used to block any other spawns and make progression very easy, provided you can avoid them.) Milkyway Weavers can pass through walls, can appear in numbers, and only take damage if you attack their heads, making them potentially annoying to deal with.
- Solar Pillar: Corites, Srollers, and Selenians. All three appear in numbers, the first performs a homing fireball tackle that deals good damage and passes through walls, the second and third perform a rolling attack that doesn't pass through walls or home in, but hurts even more, and the Selenian's rolling attack deflects projectiles back to you. And if one attempts to avoid them by flying... the Crawltipedes are there to make sure you don't. The Corite is notable as an annoyance because one can cheese all the other enemies by standing within a box of solid blocks and using a powerful, non-reflectable wall-penetrating weapon like the Death Sickle or Solar Eruption, thus avoiding Crawltipede ire and hitting all the other enemies safely, but Corites will simply fly through the box to hit you and are very resistant if not outright immune to knockback.
- Vortex Pillar: Storm Divers can fly around, and fire a very painful shotgun attack that can be tricky to avoid. Thankfully it's easy to hide from them behind terrain and use a penetrating weapon to beat them. They can also only fire their shotguns horizontally, so flying above them prevents them from taking aim. Alien Queens spit a projectile that causes the Distorted status, causing the player to float about uncontrollably... which also makes them easy targets for Storm Divers. Finally, Alien Hornets can sometimes be spawned right on top of you by the pillar for cheap Collision Damage.
- Master Mode added in the Journey's End arguably turns every monster in the game into this, giving all of them 50% more damage and health on top of what Expert Mode already adds on. To add further injury, progressing the world into Hardmode further increases the enemies' life pools, giving them more health than a Post-Plantera Expert world would give them.
