I learned this trick with the "suicide bomb" spell (forget the name, but it a chaos spell):
Get some gryphons. Put 2 in your army and leave the rest in reserve.
During the first turn of battle, move the 2 stack of gryphons towards the enemy. Split the gryphon into 2 stacks of 1. Now the 'new' gryphon can move to the other end of the field.
Cast the bomb spell on him, move him next to all the enemy troops. Cast the swamp (rage) shield on him. Now tell him to defend.
Watch as all the enemies focus him and surround him...then blow up
Repeat this as necessary. Use inquisitors to raise the dead gryphon too !