There are a lot of posts here, so I may have missed someone saying this exact thing.
I use, for my water, shrink in the top priority and sleep as second priority.
On the 4th round with the 1st mob, the controller will see that the mob is already shrunk, so the second priority spell is cast. On the 1st round with any additional mobs, the controller sees that the mob is not shrunk and casts shrink.
I have been doing GST for a while tonight, and when I made that switch, I noticed that the higher damage done to the 2nd mob added a small amount of EXP and with shrink being less SP than sleep, you're getting a little extra help in that aspect too.