In the context of a union event, if I have the spare AP, I would try to reroll via a lilim fight if possible, maybe the 8 AP if not. 'spare AP' here means that I'm still on track for 100 AP by the time of the nightly gem cave farming period (I really look forward to the next gemcha update to remove the need for this). I'd rather not spend extra seeds or half-elixirs just to reroll. (this inclination contributes to why I'm sitting on 2k half-elixirs and 6k seeds)