Interesting question. But ofcause it depends on the lore you've chosen and the enemy you're facing.
-If you're facing an army with the capability to single out characters and fry them, hide your wizard out of line of sight behind a friendly regiment or hide him/her inside a large regiment.
-If he has long range damage spells, then keep him at the back (handgunners isnt a bad choice)
-If he has area of effect spells, then keep him in a large melee regiment or behind it (to keep him out of line of sight)
Generally i'd keep him away from a horseback (unless you need him to travel fast around on the battlefield, say if he has aldreds casket of sorcery). Wizards wont do much harm with a unit of knights (unless you've been lucky and rolled a nice meleebuffing spell), and putting him in a unit of pistoliers/outriders is abit risky imo, as these units already are really really squishy.
Anyway i've got a anecdote i wanna share.
I was playing WoC vs O/G, and was completely outnumbered and even outflanked. I ganged up my units of warriors and steeds in a tight formation, hiding my chaossorcerer from line of sight between the units, then steadedly marches towards the center of enemy formations, while deflecting wave after wave of greenskin units with my Hellshriek (i think thats what it is called. Its a 18" area feartest) until my units could finally clash into his general, killing him in a bloody duel - and victory was certain!
the morale of the story being: 1) chaosmagic is awesome 2) positioning your wizards is very important :sinister:
- Tash