I know Farmer Nishi is quite popular for leveling pets, but I've moved on to Courageous Yon, and it feels so much easier.
1) None of his pet AoE's, so no risk of seeing your leveling pet die while he's in the back row
2) Only the 3rd pet (goat) has a little heal, but nothing like the repeatitive heals of Nishi's two elemental pets that sometimes makes the fight drag a bit too much
3) Wether he starts with the hare or the chicken, his first move only hits for under 200, so it's easy to get a pet very low level in there and move him out on the 2nd round, and he'll be safe in the back row
He has a critter, a beast and a flying, so it's easy to find pets good against them. Flayer Youngling and Celestial Dragon, two very popular pets, are more than enough to take them down. I'm sure there's some pets that could solo all three.
Anyway I find the experience a lot smoother with Yon than with Nishi, thought I'd share