Hydras are always a lot of fun for damage output. A twelve-headed Cryohydra is a lot of fun.

You could also get a Young Adult Blue Dragon. Frightful Presence is a lot of fun, and immunity to electricity is a lot of fun.

Shambling Mound + Energy Substitution (lightning) Persistant Cold Snap is lulzworthy.

Your problem is that most 'bruiser' monsters tend to have a LOT of HD to make up for the fact that a party can dish out a lot of damage.