Put on the perk on a stormcaller that grenade kills recharge melee and melee kills recharge the grenade. Load up the siege of the warmind mission get to the part where a bunch of thrall keep running at you before the yellow knight. Get a grenade kill, then a melee, once all of the thrall are dead jump off the ledge and repeat. I used this method and finished in about 10-15 mins.