With the special abilities ruled out for being 20+, that only leaves the double buffs as being potentially good.
Worms have stats and versatility, which would make it better than the boar as far as buffs go, but loses out on charge.
Water striders have spell power and crit for a decent combo, specially if someone is already providing versatility.
Chimaeras have multistrike...do they have their special that slows at 19? Would make them better than the dragonhawk.