I'm aware they pop at the beginning, but they also pop at 28:00 and 26:00 for the full 12, since you need to wait for all 12 to come out before you can actually start fighting them.
We used Fire cause at the end of the day I don't think it really matters, I know some Mandragoras take high Fire damage (wiki seems to verify they take high fire damage?) but at the end of the day, both Wind and Fire will probably suck against it unless you have Gambits + Raykes flying out. We had no issues hitting 99,999 100% of the time with bolster cause it wasn't Lightsday for once. I think I was pulling off 75-90k without bolster.
XI Wiki


