99Vidas (EP)

Sadiv66 will be the last character you unlock.

In order to unlock him, you must beat the game with every other character in the game.

If you want to speed up the process, you can play through the game with one character and on the final boss (when he is almost dead) you can use extra controllers to play co-op as any characters you haven't beaten the game with. When the boss fight is over, the game will count it as beating it with each of those characters.

NOTE:
Using this strategy caused the game to get a bit buggy following (has trouble loading your save file for about 5 minutes or so). Fortunately, it sorted itself.

Once he is unlocked, you just need to use him to beat the game for the achievement. I would recommend playing on Easy and then playing New Game + all the way up to Elite to get that achievement as well.