Yes, you can FLB Eidolons as soon as that element's Guardian+ comes online. I think it requires a whopping 200 drops from that boss, and the droprate... well, we have no idea. Could be 1 per fight. Anyway, once FLB there is an extra effect when equipped to sub-slot.


All gacha Eidolons get this FLB treatment, by the way. I've added it all to the encyclopedia in Sanahtlig's toolbox, which is still very much a work in progress, but it's slowly catching up to speed.