Do it this way, and you'll never take a hit from a harl.
Pre-fight: All-int elf wiz preferred, level 150, wearing at least Nightmare Helm and Amulet of Free Will for 100+% Mysticism resistance.
Before you jump a harl, at least have extension cast. (I like to go full invis, regen, invul, etc. in case I screw up.)
Have less than 60% encumbrance so your Freeze sticks before you get hit.
1. Cast Freeze on harl.
2. Cast Hold Monster through your AoFW until you stick one. If it doesn't stick in 9 rounds, recast freeze and start again.
3. Let's say you stick a 6-round hold. That means you have 5 rounds of free action. First round:
3a. Unequip AoFW. (This lets your Illusionary Foe spell work 100% of the time.)
3b. Cast Illusionary Foe 2-3 squares away from the harl.
3c. Cast Ill Foe twice more in the same way. If the harl casts through freeze your copies should kill it.
3d. If the harl is still alive, reequip AoFW and go back to #1. Rebuff or refreeze while his MR wears off. This is why you don't want extended hold on him... you just have to wait longer if you don't kill him. Two copies are more than enough to take care of him.
4. AFTER BATTLE, AS SOON AS YOU LEAVE CLOUD, RE-EQUIP AMULET OF FREE WILL. Forgetting to do this is the downfall of the strategy!
1. Freeze Harl. If it wasn't an extended freeze, cast extension next round and freeze the third round. You should have a reliable 12 rounds of frozen harl.
2. Unequip AoFW.
3. Cast Hold Monster until you stick it, and follow the steps above.
----------While you're not wearing AoFW, you're 50% vulnerable to being mysted. Don't worry about it. Your extended freeze will outlast the worst a Harl can myst you. Here's what to do if you get mysted.
4. IF YOU GET STUNNED: guard for 6 rounds until the stun wears off, then re-freeze the Harl and start again. You'll have 6 rounds of residual Mysticism resistance in which to Hold Monster the harl which should be more than enough to stick the spell. Best of all, your first round of free action can be spent summoning an additional Ill Foe, since you don't need to unequip AoFW in order to get your Ill Foe casts off 100%.
5. IF YOU GET CONFUSED: it will be for 12 rounds. This can still outlast your extended freeze, and best of all, some of these 12 rounds you'll still be able to function normally. If you're really scared, just freeze until the confusion wears off, then buff etc. in case the harl somehow unfreezes and starts hitting you. Once the confusion wears off, follow the strategy normally. Except don't worry about re-equipping the AoFW because you've got 12 rounds of myst resistance.
The linchpin of this strategy is remembering, THE SECOND YOU LEAVE COMBAT CLOUD, to check your inventory and re-equip AoFW if it isn't already. You have a few seconds to do this before you get jumped by something, and you will get jumped! As long as you've got full Mysticism resistance, you'll be able to flee first round no problem.
The Tech Method works because a harl will either attempt to attack (50% chance) or myst you (50% chance). If he mysts you, your Nightmare Helm reduces his chances of sticking a spell an additional 50%. This means you've only got a 25% chance of being mysted by a harl, who, if you've followed the steps properly, is either frozen and powerless long after your stun wears off, or re-freezable while you're confused. So 3 out of 4 fights will go quick, and one you'll just have to wear down being mysted. It's important to note that although these combats run many rounds, they go very quickly. It's possible to freeze round one, stick a hold round two, unequip AoFW round three, ill foe round four, ill foe again round five, and watch your harl drop in round 6. This happens in about 30 seconds!