I have not had much luck with leveling a pet vs. Tommy.
There are three big issues:
1) The carry pet has to actually hit Tommy's dino, which exposes the carry pet to a big hit
2) Tommy's pet hits the back line twice per fight (in my experience) with a back line attack
3) Your other two pets need to be able to *reliably* take down Tommy's dino
So far, my attempts to carry a pet have failed most of the time - either I outright lost, or the carry pet died. The only 'success' was carrying a 20+ mechanical - high enough level to survive, strong versus beast, and self rez. But some mechs don't have strong anti-beast attacks. Other than that, a magic pet (incoming damage capped) might work as the carry pet.
I carried a pet once versus Chi-Chi, but I don't remember how much XP I got.
Turn 1 - Carry pet hits vs. Tranquility
Swap in Voodoo Figurine - main thing is to how Wild Magic up, then Rot when the death passive happens
Swap in Giant Sewer Rat - Stampede
Could possibly carry a pet versus Yu'la.
Turn 1 - Carry pet versus Emerald Dream
Mr. Bigglesworth comes in and does his solo routine versus Yu'la
Strong humanoid as a backup, such as Kun-Lai Runt or Anubisath Idol
I will keep you posted...