Glad you're enjoying the game! There are many ways to beat Morrigan. If you recruit Vega, it already makes it a lot easier. Another strategy is recruiting at least on Goboy and one Gogirl, so you can combo Amity + Cheer and give a crazy boost to someone's ATK. Another way is, after killing all minions, waiting for the parts of the song where the screen flickers blue and the Kreepers act. If you can attract them to Morrigan, they will kill her quick. In all scenarios just make sure you keep Superman safe. There are possibly other strategies too, depending on how you build your party so far.