Yeah, if you can manage to burst him down, then Medea Lily is basically going to be pretty defenseless on her own. She can keep Forneus alive, but only for so long, and Forneus is the real danger with his AoE attacks and deadly ultra-move.
If you want a recommendation for who to level, Ushiwakamaru would be perfect for this situation if you have her. She's a rider who has class advantage over both Forneus and Medea, and she's single target so she's going to deal more damage for bursting demonboi down compared to an AoE. Actually, she's really good in general and is up there competing with Rider Kintoki, who is known for being absurdly good
Medusa (Rider version) is also a solid choice, she's AoE but still pretty strong and reliable for farming as well. Those two are probably the best riders available in the friendpoint gacha, I'd think.