Mobs regain health very quickly in sheep (about 10-15%/sec) also so it's not good for taking a breather midway really. The wander range isn't far, only a few yards, not far enough to bring more mobs.
Allen I think your sheep is 30s, it tops out at 50s for the highest level spell. you can also re-sheep mobs if you need more duration.
It's 1.5s cast time not insta, but cheap on mana.