One week after Venusaur, Mighty Blastoise has arrived! This was is a pretty hard is you bring in a physical attacker due to Iron Defense bringing up Blastoise's Defense to +6.

I ended up bringing in a Volcanion. With Water Absorb, you don't have to worry about Hydro Pump, so you basically just need to use Sunny Day and Flamethrower to take it down. I tried using Haze in the beginning, but it was pointless since Iron Defense would bring it back up to +6. I used Flamethrower, but Weather Ball would have been slightly faster since it has 100 BP over Flamethrower's 90 BP. Here's the setup:

Volcanion @ Shell Bell
EVs: 252 HP / 252 SpA
Ability: Water Absorb
Tera Type: Fire (or Stellar)
Modest Nature
Sunny Day
Flamethrower
Haze
Weather Ball

If you don't have a Volcanion, Vaporeon might be another good option since it can learn Sunny Day, Weather Ball, and Calm Mind. For the fourth move, you might want to use Haze or Mud Slap.