Next time, go to hell (heh, I don't mean that in a rude way) only when your party has over 140 hp per member, high levels, everyone has full mage and cleric spell trees (or at least cortu), and everyone is a fighter class (fighter, lord, samurai, ninja).At the beginning of a battle against many powerful spellcasters, have three people cast cortu (cumulative magic resistance) while your front row fights. Kill off those that can cast palios to dispel your cortus (pit fiends, incubi/vile men, etc). Almost every spell the enemy throws at you will have no effect, while you take care of the enemies that can erase your spell resistance.
My argument for an all fighter-type party (after learning all spells, of course):
The toughest monsters can easily reach, hit, and critical your mage/cleric/bishop in the back because of their bad ac, regardless of how high a level they are. Hence the need for all fighters. The best armor a bishop or priest can wear will not help him/her avoid the attacks of high ninjas/cacodaemons/dark lords/nether demons/lala moo moo, etc, from whom each hit has a great chance of instant death. Boltac can identify; money is plentiful.
If you take casters into hell, use bamatu and mamogref as needed, though this takes away valuable opportunities to cast cortu.
Finally, remember that nether demon breath can critical, and has a much greater chance of penetrating magic resistance than most spells. Consider casting mahaman w/ two members, hoping one will get the option for complete resistance. Drained levels are easy to make up.
W/ these tips in mind, you should never lose a single member in hell again.