The main sources of sustained damage in this game are assault skill, eidolon passive buffs, defense down debuffs, and elemental advantage. Ideally, you want to leverage at least 3/4 of those. Attack buffs are not only transient, but they're additive with assault skill and therefore their effectiveness doesn't scale well into late game.

The main problem with elemental disadvantage is that it multiplies damage on your team, which is deadly in Ultimate difficulty and higher. Typically you don't want to deal with elemental disadvantage, no matter how strong your main team is.