I’ve been doing this on a couple of alts for years. In my experience, it’s best to also occasionally do some other activities that make the character stronger.

For example, when I reached the point where I wanted to move a pet battler character to Deadwind Pass, to battle Lydia Accoste and the surrounding wild Arcane Eyes every day for a few days, I got killed going into Deadwind Pass.

Which was a matter of my gear being very low level, and some of my best skills being unlearned.

I have never done it entirely with pet battles, but definitely hit gaps where I just focus on doing the pet battles and it can end up lasting for several levels here and there.