Keep in mind that whoever you are trying to assist has to have auto attack
on. If someone is targeting a creature but isn't in melee you won't target
said critter(like a wizard casting spells on a creature but not actually
attacking it with a weapon). This doesn't really matter when you are using
your tank as the target for assist but if you are trying to help your mage
friend out he may or may not be using auto attack.

Yours,

~Aereuss Ravins of Tarew Marr