88 Heroes

You unlock this achievement after beating the final boss after room 88 on 88 Mode.
If you (like me) have trouble reaching the last room, this is the way I did it:

In 88 Mode you have 88 lives / characters.
If all lives are gone it's game over, but you can use a continue with the last character you used.
So if that last character is for example Bat Bot.
You can finish the game only using that character.

- Start a new game and advance to room 03/88
- Keep killing off characters untill you get Bat Bot.
- Finish the room using Bat Bot (Don't let Bat Bot die)
- On the next level with the new character continue killing off the rest of the characters
- Your last remaining character should now be Bat Bot, and you can now try and finish all the levels

Tip: If you pick up coins you revive (random) characters.
So don't pick up any coins because you wan't to keep on playing with Bat Bot only!

Using this methode I also unlocked the Nick of Time achievement.