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I'd say Eye of Tzeentch or Collar of Khorne are usually a given for the Tzeentch-enhanced protection.
Or give him the banner or the gods and make your units that need to be stubborn, stubborn whenever you want.
Get 1 wound on a silver helm and it already earned itself back. :]
Also, i Prefer halberd over flail
he can use a flail, halberd, great weapon, magical weapon, whatever he can legally take. You won't get any bonus for for 2 hand weapons and you'll only get a plus 1 str bonus for a great weapon. I prefer the flail because it can pop chariots and if you can hit, whack heavily armored knights and what not easier. You probably don't want this guy in combat for a prolonged amount of time otherwise you should get a magic weapon and shield (should have a shield anyway to deflect arrows, handguns, etc.)
I find bloodcurdling roar useful, depending on what the rest of your army composition, I would definately get him some protection from shooting and magic but it depends on the role he needs to play as to what magic items he gets.
Indeed only, a great weapon is nerfed on a disc.
further any ward safe item is good.
You have morrslieb, for a safe vs alot of shooting and in combat. Colden eye, versus shooting and magic. Collar, nice alround. 5+ safe vs anything and MR2.
also the book of secrets often pops up, so you can have a 1 dice flying fireball.
Yeah flail is better, you want to pick your battles and obliterate whoever you hit. Especially chariots!