Character attack vs. elemental attack
The damage formula treats character damage and elemental damage as separate terms. The formula look like this:
(1 + %character_attack) * (1 + %elemental_attack)
Sources of character attack include Assault and Pride skill, some eidolon passives, and most temporary buffs. Sources of elemental attack include eidolon passives, elemental advantage (+45/48%), Passion-type Relic weapons, and some temporary buffs (including the Kaiser Dragon summon ability).
Maximizing damage generally means raising these terms in a balanced manner. Since most developed teams will already have high character attack from stacking Assault weapons, primary and friend eidolons with elemental attack are usually optimal.