Here is my pet levelling team and a TD script to use it. I run this against the Icespine Hatchlings in Frostfire Ridge. It typically takes them out in 7 turns and I can run it at least a dozen times before needing to Revive/Bandage. The only caveat is that your levelling pet needs to be level 3-4 before you start.
1: Rotten Little Helper (1,1,1)
2: Leveling Slot
3: Rotten Little Helper (1,1,1)
For help with scripts, see Xu-Fu's page below.
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