[Baalz]

Well, undead horsemen are pretty darned good in such a situation, I have a hard time thinking of much that is better for exactly the reason you point out. Against a large number of charge bonus lances there's not much that is gonna be immune, the best bet is to have a couple bodyguards (or summon some fire elementals before doing anything else) to keep from being swarmed. Outside of that, as I mentioned Pheonix Pyre will give you a second chance if you do get caught with your pants down.

If you're fighting tough guys Heat from Hell might be a better choice than Firestorm (though they're not mutually exclusive if you've got the casting power) because it'll stack with the heat aura and doesn't matter what their protection or hitpoints are. Heat from Hell plus a couple castings of living fire followed by your fire king wading in and it should be almost incidental if you inflict damage... everybody is gonna pass out from the heat and never wake up - particularly those in the frontline trying to damage you and in the glow of several overlapping heat auras.