My suggestion is to use Frostwolf Ghostpup. Scratch hurts the undead very badly, haunting song to heal your team or refuge to protect the main pet + any critters, and ghostly bite as a strong move to use right before swapping the pet. Considering they like aoe with graves, and bone serpent seems to be there to counter healing, I think refuge may be the right choice in case they use call darkness + nocturnal strike, as the frostwolf pup is faster.
Depending on what the 3rd pet he brings, I would suggest an aquatic pet that will take less damage from undead moves, and can change weather away from darkness, as well as not take increased damage from beasts like an extra critter would. The ideal pets for this would be the Blue / green / red / white tiny carps, as they can make the weather advantageous to aquatics. The weakness with this is he may try to counter by using a terroclaw hatchling to get in more flying damage on the aquatics.
In that case, your best third pet would likely be a dragonkin, or non aqautic. that can deal arcane damage or aquatic. You have a few choices here, however I would advise a Nexus Whelpling or Curious Oracle Hatchling . Because the teroclaw will swap to elemental and take aquatic damage, you may want to use a non dragonkin with aquatic moves. Nexus whelpling has arcane winds + Mana surge to deal large damage to any pet he has out ( except for teroclaw dodging). Curious oracle hatchling is good on a couple fronts, but terrible on others. It is humanoid, and thus will die quickly to aoe from graves without healing or graves dying before a second set of aoe gets off. However, it has water jet to hurt the elemental teroclaw, and backflip to interrupt graves or bone serpent because COH is faster.
Another good 3rd pet is Spirit Crab, an undead with snap, surge, and shell shield. It has very high hp, so the shield is useful against small damaging moves like the bone barrage. The surge makes this pet faster than the potential teroclaw hatchling, which means it will for sure get damage off in case of dodge. The snap is a beast type move, so it is not particulary good against any pets he will likely use, but it is decent.
The combination of Frostwolf Ghostpup, a Tiny carp (my preference for this situation is white, it has dive and can avoid nocturnal strike, or just avoid any hit and deal decent damage, cleansing rain as a small heal but gets rid of the call darkness effect and buffs aquatic damage. You can use Water jet for some good damage too), and Spirit Crab or any good 3rd pet you can think of, should be able to fight off the specific combo you mention. (as a side note, I would encourage trying graves as your third pet if you have him, as he would make an excellent clean up pet after their graves is dead)
My advice is to start out with frostwolf ghostpup and use scratch to end Graves or bone serpent quickly. Their strength lies in the fact few run pets that can actually kill either pet quickly, and getting them to their last round of life faster means you take less aoe damage. Ghostpup can end either within 3 rounds by using scratch. scratch, ghostly bite. The stun won't matter much because they will be "dead" or you can swap to another pet to begin countering the next one.
Finally, if he is killing your ghostpup with the vengeful porcupette, You may need to bring in a mechanical that will deal with it. Iron starlette would work, or darkmoon zeppelin, and a few others. the iron starlette can combo any of his pets down with the wind up + supercharge + wind up combo.