Soul Hackers 2

Demons are fused at Cirque de Goumaden, which is basically the only thing in Roppo Realm bar some side quests (and after a point, a shop). You don't get access until you clear Arrow's Soul Matrix 1F, which you need to do as part of the main story. Once you have access, select Fusion -> Normal Fusion, then any 2 demons in your posession and to get a stronger demon and this achievement.