[Aerich]

1) Give him the best armor/armor-enhancing items your party has, and a ring of Free Action.

2) Use movement-inhibiting spells (Grease, Entangle, Web, Slow) on melee opponents; you can set up a couple of these spells and walk forward through them with your Free Actioned tank then back when you draw some enemies.

3) Use summons-fodder to give your enemies some other targets in tough fights.

4) Cast Protection from Fire on your tank, draw some enemies, and Fireball/Flame Strike away. Use fire potions (Fiery Burning, Explosions) as well, even if you have to buy them.

5) If going up against good missile troops, use Protection from Normal Missiles; this is a tank-saver in the Severed Hand and against the tower archers in Lower Dorn's.

6) And I second the previous posts, especially about the buffing spells. Protection from Evil 10' is a spell I cast every day, right after resting. Long duration, pumps everyone's AC by 2, and protects from Charm spells. Use Defensive Harmony and potions before/during hard fights. Haste is good too, although the necessary rest afterward can be problematic and cause delays. Make sure you heal up your tank (to within 7 HP of full) after every fight, no matter how minor you think the next fight will be.

7) If you have the Boots of Speed or Boots of the Fox, give them to your tank. Then you can run around madly as a decoy in front of your party while everyone fires away. The extra speed should enable you to avoid the majority of hits by melee creatures. Cheesy, but it works.