[3.5] The Bonded Summoner?

[ericgrau]

An elder elemental only has a CR of 11.

This. Once per day you can bump your dynamic duo up to CR 13. For a brief period of time, 1/day, you reach par with a non-caster. Then you hit level 14 and become worse.

Um, mage armor helps I guess. All the other buffs could also be applied to the party fighter/etc.

EDIT: Oh, I know what might break it. The elemental has a better BAB than the party fighter until you hit level 18. Polymorph it into something else entirely.