There are some achievements in this game that are obscenely difficult to get, if not outright impossible without RNG favoring you.
- The No-Damage Run achievements. At least they're mercifully divided into groups, so you don't have to go through all eight in one run.
- The king is the Dark Boy (Depths) No-Damage Run. That level set has just the right combination of pits, Demonic Spiders, Goddamned Bats, indestructible moving spike blocks, and cheap-as-hell bosses (plus Mom) to make it an absolute nightmare to get through, no matter how absurdly overpowered your character is. Even the Womb is easier because it does not have pits. Rebirth made the Womb somewhat more difficult by adding pits.
- Rebirth ups the ante on that one with Dead Boy, achieved by completing one of the game's Brutal Bonus Levels, The Chest or the Dark Room, without taking damage. Here's hoping you packed a Holy Mantle... though it is slightly easier on The Chest, as four free (as long as you have keys) items are available from the starting room, and there's a chance to find more after completing every vicious room.
- Rebirth has The Lost. Unlocking it is a massive Guide Dang It! and a Luck-Based Mission for the first part, though you can mercifully use seeds. Then, once actually unlocked, you have to do no damage runs of the entire game through both Brutal Bonus Levels (Mega Satan is optional) and the boss rush, and then do it all again on Hard mode to unlock the Godhead.
- Thankfully, Afterbirth made the unlock easier: die in a Sacrifice Room while holding Missing Poster.
- Assembling Super Meat Boy and Bandage Girl in Rebirth. In vanilla Isaac and WOTL, assembling Super Meat Boy wasn't too difficult to do as long as you got the Book of Revelations early enough. And you didn't have to assemble it to get the achievement, just kill all 4 of the Harbingers in one run. Unfortunately, the Book of Revelations no longer guarantees a Harbinger boss fight, instead giving a higher chance to run into them. The real kicker is that they now drop a Cube of Meat or a Ball of Bandages, so you have to hope the first one drops the item you want. Running into the Headless Horseman or Conquest will also ruin your chances of getting the achievements. Counter-intuitively, these achievements are actually easier if you go out of your way to avoid unlocking Conquest (which requires getting ten Angel Room items). Once Conquest is unlocked, it becomes downright impossible to manage a fair playthrough where you get the whole set. It's considered a lot less luck-based just to set up a way to infinitely recharge the Monster Manual to get a level 4 Meat Boy/Bandage Girl. It says something that these achievements have a lower completion rate than almost every other achievement in the game, second only to the shop unlocks and all of The Lost's achievements. Mercifully, Afterbirth made this incredibly easy to earn with the addition of Greed mode and Lilith. Lilith's active item, Box of Friends, duplicates any familiars she has for the room it's used in. With Cube of Meat/Ball of Bandages, the effect is an immediate upgrade with each use. Greed mode's wave mechanic allows you to use the box at least three times in a row, since the charges accumulate with each wave. In tandem, this means you only need to find one of either item to get the associated achievements (though ball of bandages doesn't appear in any greed mode item pool, so good luck with that one.) Boss Rush or a 48 Hour Energy pill in the main game will also work. Afterbirth+ went a step further and added the Potato Peeler, an-infinite use spacebar item which removes one red heart and generates one Cube of Meat along with a damage increase. Four hearts and you've got it.
- Collecting all of the items for Platinum God and Real Platinum God in Rebirth is far more annoying than in the original. This is mostly due to two reasons: the Secret Rooms and Steven. Secret Rooms have a much greater variety now, most of which do not contain items, which combined with a larger number of exclusive items means it is that much more difficult to get everything from that pool. Most resort to Loophole Abuse with the D6, Judgement cards, and mushroom drops. Steven was originally a nightmare to find in Rebirth due to only being a rare boss drop, but Afterbirth has since fixed this by putting Steven back into the golden chest pool.
- Two achievements in Afterbirth+ depend on Daily Runs. 'Dedication' is for participating in, but not necessarily winning, 31 Daily Runs in a row, which is generally regarded as a month-long chore and impossible for those who don't have access to a computer every day for a month. 'The Marathon' is for winning 5 Daily Runs in a row, albeit they don't have to take place on consecutive days. Given that some Daily Runs task you with beating Mom with an easy character, and others require you to make it to Mega Satan with The Lost, it's not uncommon to find threads on the Steam forum warning users to stay away from a particularly bad daily. To make things worse, while they only unlock a trinket and a pill, both of these are necessary to unlock the 1000000% completion achievement, meaning that you could have beaten every challenge and every boss with every character, but if you can't take part in 31 Daily Runs in a row, you'll never unlock the final achievement in the game. Thankfully, a later patch made it so that the 31 Daily Runs didn't need to be consecutive, removing Dedication from this territory.
- The achievement ZIP! requires beating the Lamb in under twenty minutes. Just getting to Mom on a good run takes nearly twenty minutes. Rushing through the Womb, Sheol, and the Dark Room on top of that basically requires having found the Treasure Map and Compass so you waste as little time as possible. While there's the possibility of using a Sacrifice Room in order to teleport straight to the Dark Room, this is very much a Luck-Based Mission. You not only have to have one spawn on any floor up to the Depths, you have to be able to take at least 12 hits on that floor (if you're lucky, the Sacrifice Room will spit up 7 Soul Hearts to make that easier, and you can go back for health pickups) and beat both mandatory Angel fights in the process. Once you make it to the Dark Room, you better hope you have good enough items to track down and kill the Lamb in a reasonable time frame. In short, a lot has to go right for this achievement to be remotely possible.
- It's The Key requires you to beat the Lamb without ever picking up any hearts, coins or bombs throughout the entire run. This is hard enough as is, but it's entirely possible to fail through no fault of your own. Pick a character that can't fly? Have fun resetting when the RNG sticks you in a room with only one path and coins smack in the middle of it. Consumable blocking a door or hatch to the next floor? Too bad.
- Thankfully, there is a way to make this achievement slightly easier. By inputting the special seed "GONESOON" you unlock a modifier that causes all pickups to vanish after a short period of time, which helps prevent being trapped in a room. Unlike some other special seeds, this one does not disable achievements since it is not usually an advantage. Repentance also added Tainted Cain among the playable characters, who can collect pickups with his bag without disqualifying you from the achievement, though you still need to be careful to not accidentally pick them up with Cain himself.
- Unlocking the Forgotten makes the Lost look like a cakewalk by comparison. First, you have to beat the first boss within a minute, which can be hit or miss. Once you've done that, a shadow of a shovel handle appears in the starting room. Bomb it to get the Broken Shovel. To get the other half, you have to beat the Boss Rush. Sounds easy, right? Wrong. The Broken Shovel is essentially the Toenail trinket on steroids. Two Mom's Foot enemies will attack you constantly, and activating the Broken Shovel only serves to remove them from the current room. It has a four-room charge. This means you have to rush through a fair portion of the game under constant attack. You can't cheese it by picking up another active item on the floor, either; ditch the shovel and it disappears if you leave the room. On the plus side, the feet will kill your enemies and break bombable things for you, if you can trick them into it, and having Pyromaniac/Host Hat renders them harmless... until Repentance, where Mom's Foot no longer counts as explosion damage, rendering those items completely useless.
