Aquatic Battler of the Dragon Isles

Use Aquatic pets that have Beast and Critter abilities to counter Arcantus, who is a Magic pet. A team of Chitterspine Skitterling (2/2/2), Bloodseeker (2/2/x), and Chitterspine Deepstalker (1/1/x) works well: open with Black Claw, then Swarm until Chitterspine Skitterling dies, bring in Bloodseeker and use Bubble followed by Sneak Attack four times, then finish with Chitterspine Deepstalker's Skitter if needed.

Use Aquatic pets with Magic abilities such as Slimy Eel (1/2/1) and Slimy Fangtooth (1/2/1), paired with an Aquatic pet that has a Mechanical ability like Hydraling (x/1/x). Open with Moonfire, then Sewage Eruption, then Ooze Touch — Slimy Eel will likely die, at which point you swap in Slimy Fangtooth to continue.

Use Swog Ghost Shark (1/1/1), Chitterspine Skitterling (2/1/1), and Chitterspine Deepstalker (1/2/2). Open with Ghost Shark's Huge Sharp Teeth, then Blood in the Water, then Dive; swap to Chitterspine Deepstalker and use Black Claw, then Fish Slap until Chitterspine Deepstalker dies, swap back to Ghost Shark and repeat Huge Sharp Teeth, Blood in the Water, and Dive before Swog's second Pump Up. Note that only Ghost Shark may be faster than Swog, so some RNG is involved.

You do not need the associated world quest to be active for a battle to count toward this achievement — defeating the trainer or elite pet with a full team of level 25 Aquatic pets is sufficient regardless of world quest status.