By beating the sacred grounds and getting the best ending with every space in the inventory full (excluding weapons), including the medals, the items important to access the sacred grounds (iron bond and booster 2.0), and not ever using the life bottle.
Are you sure that you have Santa's Key? You're supposed to hand it over for the Fireball. Usually changing emulator or core also works in case an achievement isn't unlocking.
I meant to say you're not supposed to have the key still in your inventory after you give it back to the mimiga in Grasslands. If you still have it in your inventory then that's the issue (strangely enough). Otherwise, use another emulator/core. If you can, dm either here or on discord if needed.
Hey, I noticed your comment on the Twisted Metal: Head On set. When you say "wipe your save" do you mean the entire gamesave, or just the save files for story mode? Because there are three characters I have beaten the story with like 20 times now and their achievements just will not unlock, no matter what I do. :(
From what i recall, the ending achievements triggers when you do not have the endings already unlocked, so it has to be the entire save. You don't really need to wipe your save, you can just move the game's save (if on retroarch, should be in saves > PSP > SAVEDATA) somewhere outside of the save folder temporarily to play the game with a clean save (of course making sure to not enable cheats on accident). Then when you are done, when you got the achievements, put the original save back where it was. If you're still unable to get the achievement after this, i'm afraid you have to talk with the developer of the set about it.
User Wall
- Map system, Silver Locket, Arthur's key, ID Card, Santa's Key, Rusty Key, Gum Key, Jellyfish Juice, Chaco's lipstick, Life Pot, Clinic Key, Tow Rope, Curly's Air Tank, Curly's Underwear, Whimsical Star, Arms Barrier, Alien Medal, Nikumaru Counter, Teleporter Room Key, Sue's Letter, Booster v2.0, Little Man, Mushroom Badge, Iron Bond, Clay Figure Medal.
Usually changing emulator or core also works in case an achievement isn't unlocking.
If you still have it in your inventory then that's the issue (strangely enough). Otherwise, use another emulator/core.
If you can, dm either here or on discord if needed.