Even if a boss uses a turn to negate them, that's a turn it's not using to hurt you. If you get stuck, try fusing new demons to take advantage of the battle system. Press turn makes party composition much more important than level in terms of progression. Tyrant, Vile, Fury, Fiend, Seraph (Herald in future games), and Deity class demons are also generally among the stronger demons you can get. Not all demons can evolve, but those that do are generally among the most powerful demons you can get at the time. 'Signs of change' means a demon can evolve, and that's always something you want to do. Demons will in fact become obsolete, and need to be fused. However, don't get attached to your demons and try to keep them around all game. So it's in your best interest to level up your demons and get them to learn as many skills as possible before fusing them.
When you fuse a demon, the number of moves that can be passed on is determined by the total number of skills known by all demons used in the fusion.
