It depends entirely on your build and what your units are intending to do. Triangle Adept users can go +Atk to push out a few more one-hit kills (since killing in a single hit avoids the risks of being hit by most special skills) or a defensive stat to bulk up against specific opponents. For example, both versions of Tiki can use +Res to help tank Julia and Dierdre a bit better, especially since Divine Naga ignores buffs granted by Fortify Dragons. Tiki and Nowi can use +Def to help tank bo