Defending them is by far the hardest part. If you set everyone to watch each spawn (two on left, one on ramp on left one near the wall cave left) two near the middle killing adds coming up from the middle and supporting the sides and than two on right (one near cave watching ramp near Templar and the other on the ramp on right side near where the tower is killing the spawns coming out of the cave) you should be able to do it after a few trials and errors. Also remember when he says he is summoning his legions everyone should leave the map area and hide where the goblins snipe from, you don have to fight them they go away after a few minutes. Use the time to reload, use bathroom, talk strategy etc.