Quote Originally Posted by Amak View Post
-- I know if all your eidos match the element you get a small boost --
Um, there is no such thing.

There's a hidden bonus for using the same element Eidolon; your Soul and each Hime gains a bonus of 10% stats from same element Eidolons. So a 1500 Atk Water Eidolon is actually 1650 Atk to all Water Hime and any Soul equipped with a Water weapon. You can test this by equipping and unequipping same element Eidolons in the Edit menu, then looking up their stats if you want to confirm it.

Similarly, there's a weapon bonus. Each Hime has one favored weapon type from which she gets extra 20% stats, and each Soul has two (except Dartagnan who only favors Gun types, and does NOT double-dip into that despite having two icons for Guns). This usually doesn't matter all that much since there's so many different types of weapons, but for whales who can choose their Grid this can actually matter a lot. For free players, it's usually only something to consider for Thunder - there's just SO many Gluttony Guns that Thunder Hime who prefer Guns will get a big stat boost.