Soul Hackers 2

You should be fusing demons anyway as you level up, or you will notice that combat is harder than it should be (unless you're on easy). You need to get every demon in the compendium for another achievement, although any you fight as regular enemies are obtainable via Demon Recon in that dungeon, so not all require fusion. This counts both Normal and Special fusions, so you should unlock this naturally as you play, in the first half or so of the game.