Ill have a say in this matter, jsut choose your achievements wisely and put the best ones in the gold slots.
Laharl has always ran with all 6 of them and the most benefit comes from the +10 size and 250 hp by far.
Nura and Yuki both run with 2-3 achievements and so have Shikii and Jinrou, they have been upgraded to 4-6 since they participate in the bloodgames.

Nate tweaked the cost for the last 2 achievements from 12 a week to 6 which cuts into the costs majorly and ive been happy with that, if you are doing anything about the trophies than make the third one optional for both gold and trophies but not more than that.