Spear is my worst weapon lol. It would be the sword but there's one really good sword aspect that I like a lot.
Yeah
Thesius
can be pretty tough with the spear. You could try to create an AoE cast build alongside your spear. Dionysius and Ares would be good for that. A full powered Call is very helpful in any boss battle really (some are more helpful than others).
I wouldn't stress about not being able to consistently beat that particular battle though. I thought I'd never be able to beat them and when I did I felt it was my only chance at escaping. It was not and eventually it got to the point where they're pretty easy now lol.