The first thing that I can think of is that as a new player you most likely do not have a lot of skill levels on your Weapon grid, much less a full single element weapons grid. The elemental damage increase from the Eidolon Illuyanka isn't going to give you more atk power than from your Disaster Eidolon. Elemental attack really packs more of a punch once your Weapon Grid gives you around 90% bonus to atk. If it's lower, then a Character Damage Eidolon is going to be better (when comparing same percentages). So by removing that Eidolon you lost out on Atk power and HP. For now you may want to switch back to it until you get a bit stronger and more skill levels on your weapon grid.