Very simple actually

DONT FIGHT IT LIKE IT WAS INTENDED THAT IS A STUPID IDEA, THE CRAB WILL KILL YOU

First retreat out of the area then attack the south most ship of those pirates, they wont be able to call the GIANT FICKIN CRAB in

Remember to change the reinforce position so your allied take the beating (yes yes)

Now finish the battle then go and kill the giant crab in the next one, do it manually

Blast him with your batteries