The issue with hydras is that they seem to not use their breath attack when it might hurt either you or another summon. This means that they often just bump into things, doing negligible damage. If they are summoned with a clear line of attack, though, they can completely decimate the enemy.
Also, I will agree with others that Cornacs make great summoners, even if only for the experience. I just played my first summoner and completely dominated everything, only dying when foolishly digging into the overpowered multihued wyrm vault in Dreadfell, and once more while farming jellies when I forgot about my Sudden Growth heal after two uniques spawned on either side of me. Atamathon only hit me once through my massive wall of spam, taking me down to about 400 hp, which I healed next turn. I may have missed out on some nice racials, but I did hit 50 easily, and I had plenty of generics to spread around antimagic, fungus, and harmony. 5/1/5 Fungus was critical for the longer fights. Maybe smarter playing could have ended those fights quicker without my equilibrium ramping up so high.
I decided to go for Eye of the Tiger to maximize the number of summons I could field, which worked great with a final Cunning of 120. (http://te4.org/characters/37268/tome/f3 ... 516e4148de
). The longer battles generally consisted of me spamming whichever summons happened to come off cooldown, with the occasional Rage or reposition thrown in when I hit my max summons count. The most difficult region by far was the oozenest when it got up to spamming constant uniques and bosses.