BTW, Malladine,I think that your party will be able to handle HoF. I'm not a big fan of monks and have never played one. I guess that monks are OK in HoF. I've always heard that they really come into their own in the upper levels. Guess you'll find out.
HoF requires different tactics than normal mode. In normal mode, you are uaually using your mage to fire off a bunch of area effect damaging spells to wipe out the enemy. However, in HoF mode, the bad guys have too many HP to be taken down by one fireball. You'll find that stunning, enchantment, and summoning spells are the most useful. The best area attack spells are those that do not have an upper limit on damage. (i.e. Fireball maxes out at 15d6, but Skull Trap has no upper limit.)
You need to remember that while enemy monsters have tons of HP, so too will your summoned monsters and animals. A lowly summon monster 2 will be quite useful early in an HoF game. Even later in the game that lowly monster hwill have enough HP to hang around for a few rounds and be mildly useful, if only to suck up some damage. Be careful with your high level summons. Some of them are physically large and cannot pass thru some doorways. Nothing sucks much more than having a healthy Frost Giant summoned ally that cannot go thru a doorway and stay with your party. I tended to use lower level allies when I wanted some cannon fodder to travel with my party. I saved the high level summons for big battles in more open areas when I needed more nasty allies. Be aware that your stunning spells might stun your summons. This isn't the end of the world if you've stunned enough enemies to keep the numbers managable.
Dropping a Symbol of Hopelessness on a bunch of enemies, followed by a multiround damaging spell like Cloudkill or Firey Cloud is a good tactic. Animate undead are really great in this situation since they are immune to Cloudkill. The Cloudkill won't kill those tough bad guys. But between being stunned, taking a chunk of damage each round for many rounds, being whacked by some undead monsters, and whatever other damage you're delivering from range, you'll be doing a boatload of damage over a few rounds.
Mass Dominate is an AWESOME HoF spell that can turn the tide of many battles if used properly. Do NOT have your cleric cast a Symbol of Hopelessness while your mage casts Mass DOminate. It stinks to stun your dominated enemies. Sure, if they're not actively attacking you, that's not a bad thing. And, yes, any non-stunned, non-dominated enemies will attack the stunned, dominated ones. But it's pretty darned nice to have your dominated enemies fighting on your side, not just sucking up damage. I won many a battle using Mass Dominate to get half of the enemies fighting the other half!