Gloves

Actually they do cast it, but they take a long while to do so. Maybe it's a long casting time or something, but the trick is to make it change the target of the spell while you attack him: for example, have him channel it to cast it on a summon and the go near it and attack it, making it start channeling the spell all again, this time to cast it on you, then go far from it, making him change the target again, and so on. Anyways, if you're attacking him and suddenly he starts using his ranged attack, you know it's time to run or get wooped by dw.