I managed to get to 15 stars. Got the last one by 1 starring Magic Sword of Sorrow X. I can't see me getting much better than maybe 1 starring Final Battle X and even then it only gives me extra levels on each reward. Nothing I can't earn in usual play.

Next move for me is to farm silver Solders and maybe get a Giovanni. I still have a few Mages and farming Heavy Armour isn't an option as I beat Sword of Sorrow X convincingly as 1 star, but not really enough to reliably farm it. Next event will be easier for me as I'm better equipped overall.