You shouldn't lock yourself into the dilemma that you need a hime team of opposing event boss's element to run the event.

I've run all advent events with my light team. I only use opposing element teams for raid events only because all the raid events (aside from Jorm) have been easy.

My alt account for raid boss farming is only a dark 24k atk team and can handle this event's expert with little problems. While it isn't as good as ultimate, it's enough to get you several copies of the weapon and eidolon if you keep at it. Not that I bother to grind events with that account.