One of the more successfull GoR tests I've had was as LA Pythium, the summonable immortal Hydra. I GoRed her and made the prophet. Sure, just misc-slots (I gave her a flying carpet and an anti-magic amulet), but she is a *major* pain to kill! You want to keep her in friendly dominion anyway (as she is immortal), and when a prophet it means she'll have a ton of health (and yes, she regenerates one head worth of health each round, no matter how much health each head has) and very good MR (which you can further boost with the anti-magic amulet). And if she happens to croak, so what? She is back in action next turn
With all those heads she has (and Holy Avenger), she will eventually kill anything (not to mention the poison, although any sensible being going against her will be poison immune).