Grand Master Zusshi

Both Rapana Whelk and Emperor Crab can often solo this, so I use both, and one leveling pet that can survive Diamonds back line hits.

Emperor Crab has [Snap] [Renewing Mists] [Shell Shield]

Rapana Whelk has [Absorb] [Shell Shield] [Dive]

Leveling Pet

Make sure you have and keep Shield up, on the crab I use Renewing Mists, rather than Healing Wave, it heals for a bit less but does so every round, if you keep it up. It seems to cover the big hits a little better than waiting 3 rounds to heal again.

He has the following pets:

Mollus (Critter)
- Absorb
- Acidic Goo
- Dive

Diamond (Elemental)
- Frost Shock
- Frost Nova
- Howling Blast

Skimmer (Water)
- Soothe
- Cleansing Rain
- Pump

Thus, they are weak against Beast, Aquatic, and Flying (respectively).

I've found an extremely reliable method of killing this trainer with only 2 pets (so as to get experience on the third pet) short of utterly horrible RNG over and over in the fight, but it exploits timing that happens throughout the fight in somewhat odd ways, so I'm going to lay out the fight on a nearly turn-by-turn basis.

The pets you will want:
1) Any of the crocs from the Crocolisks in the City (they're boe so you can buy one if needed): Chuck being an example, but their abilities are all the same. You'll want Rip, Surge, and Blood in the Water.
2) Jade Owl with Thrash, Adrenaline Rush, and Lift-Off. Note there are other Flying pets with these same abilities, and they will work in a pinch, but not as effectively for a few reasons:
a) Jade Owl will be fighting pets that use Aquatic abilities; as a Magic pet, he takes reduced damage from them
b) There are times in the fight you'll want to be slower than your opponent; Flyers will be faster at this point in the fight due to their racial ability
c) The final pet does not hit often but hits EXTREMELY hard; as a Magic pet, Jade Owl can't take more than 40% of his damage in one turn.

So here's how it works:

Step 1) Start with the Jade Owl and pull until the enemy starts with the Elemental. Note that he will open by rooting your pet so you REALLY don't want to start with your low level pet.
Turn 1) Enemy roots you, you Rip him.
Turn 2) Enemy aoe's your team, you Blood in the Water. Can't miss due to Rip, extremely low chance enemy will Dodge. Even if he does, just keep going, it won't have too much of an effect.
Turns 3+) Surge enemy to death.

Step 2) Enemy brings out his critter
Turn 1) Enemy will put a dot on you (mitigated by the fact you're Aquatic), you Rip him.
Turn 2) Use Surge. DO NOT use Rip or Blood in the Water; he always Dives on this turn, but because Surge goes first, you can hit him BEFORE he dives for some free damage. Anything else will go after he dives and miss.
Turn 3) Enemy will usually kill you this turn; it's very rare that you'll survive the Dive hitting. In the rare case you don't die, keep using Blood in the Water or Rip to do what damage you can, but the rest of the strategy will go on assuming you died since that usually happens.

Step 3) Bring your low level pet. Use one ability then swap in Jade Owl.

Step 4) Jade Owl is now out.
Turn 1) Use Adrenaline Rush to get the speed advantage on your opponent. This both makes Thrash hit harder and will save you from taking one turn of damage when you kill the opponent.
Turn 2+) Thrash the enemy to death. Thrash is a little random, but it shouldn't take more than 2 turns to kill him. DO NOT use Lift-Off, you're going to want it later.

Step 5) Enemy brings out his water pet.
Turn 1) Enemy will cast Cleansing Rain, which makes his Pump hit even harder, which is why this pet tends to be dangerous. Use Thrash on the enemy. DO NOT use Lift-Off or Adrenaline rush, even if the buff has fallen off. This sounds weird, but it will help in a few turns.
Turn 2) Enemy will cast Sleep on you; Trash again.
Turn 3) Enemy will charge up Pump. NOW you want to Lift-off. Sleep will trigger at the end of this turn, but because you're in the air, it will fail.
Turn 4) Enemy will launch Pump. Assuming you are slower (which is why you want Adrenaline Rush to fall off), it will miss. Lift-Off will hit, assuming it doesn't miss (the miss chance is annoyingly high), it will almost kill the enemy.
Turn 5+) The fight loses order a little bit here, but it shouldn't last much longer unless Lift-Off missed. From this point on, keep Adrenaline Rush up so your Thrash hits harder and so you can potentially dodge Pumps. Use Lift-Off any turns it's off cooldown where Sleep is going to trigger or when he's charged Pump. That being said, short of horrible RNG, the fight won't even last long enough for him to do either ability again.