you also get the friend effect even if they aren't literally your friend, so keep an eye out for the strongest effects you can find (usually the eidolons giving 100% or more increase)
that, and if you want an elemental atk increase but can't find one (like for instance, a list full of water character atk), some eidolons of different elements can give you a boost (for instance, Fafnir can increase elemental atk for both fire AND water teams). there's been times i gotten 120% elemental attack this way (my echidna and a friend both giving 60% each to my fire team)