Environeer Bert

This trainer's pets scale with the level of yours (they're the same level as your highest pet). This means that there's a very easy strategy: bring 3 level 1 Elemental pets with an Elemental attack, spam that, and win.

This fight goes well using three elemental pets all with Immolate and Conflagrate (and a third elemental-damage dealing ability, such as Burn.)

Keep Immolate up all the time, use Conflagrate whenever it is available, and use the third ability if the target is already Burning and Conflagrate is on cooldown.

If you just want to win then this set up works.

1) Singing Sunflower : Lash, Photosynthesis, Sunlight
2) Terrible Turnip : Tidal Wave, Leech Seed, Sunlight
3) Anubisath Idol : Crush, Sandstorm, Rupture

Sunlight is your first move with your Sunflower and then Turnip, then use them off cooldown. Use Photosynthesis and Leech-Seed as your next move in their rotation. Use Lash and Tidal Wave as fillers. You want to keep these two alive as much as possible to use their Sunlight ability.

Using these two pets alone will defeat Corporeal Hammer, and will have taken huge chunks of HP from Berts remaining two pets.

Use your Anubisath Idol to finish off his third pet. Use Sandstorm first, Rupture second, and then use Crush to end him.