I usually just use highest power over everything because iirc solar blast damage is based off of your teams' average power and it doesn't really make a difference what you use after a certain point anyway. For my helper squad I prefer high HP and ATK since all I care about is that none of my girls die before Edelweiss can heal them with her passive, and the high DEF/low HP units usually don't get enough defense to outweigh the massive amount less HP they have (damage is reduced by roughly 1/4 of DEF, detailed more HERE, and most of the 12-5 enemies have around 2,300 ATK, so St. Paulia ends up being able to take about 50% more hits than Cepha Lanthera). As other people have said, the main danger is the moving pests, but I don't think those are affected by defense and are just % health based.