He's strong vs a lot of pets. That said a high damage pet like a Direhorn will make mincemeat out of him. Also, I did once defeat a team that included two Ruby Saplings (same moveset) with just a Fiendish Imp.
I am having an awesome PVP run with the ancient, a Singing Sunflower and Mr Pinchy.
If Sunlight stays you just photosynthesis and swap to the back row. Keep swapping enough and you end up with 3 pets with over 2000 hit points each healing like crazy. It is slow going, but I've found that unless I get webbed and can't swap out nobody can do enough damage to beat me.
Sunlight does help your Humanoid opponents heal more each round with their racial but besides that it is a very strong team.