Does your warrior who grabs the aggro, from the extra giants, run away, and out of range? I know when I pull in D1, sometimes I get hit by the cheifs rampage, and I am gone, running away. But after a certain distance, I drop off that list.
Now your 3-boxing it, so only one left is the cleric, for rampage. Providing that the warrior is doing the rabbit approach, to pulling. Wolf approach should keep the warrior on the rampage list. That is your pulling, and just fighting the giants extras yourself, dying, and running back, or beating them.
Not sure which style for pulling your using there? That would explain why the cleric is getting the rampage.