Now first, you must acquire 2 diamond knives. Or hardened blades. Or whatever.
If you want to spawn them in, type the command:
ITEM DIAMONDBLADE 2

and bam. 2 diamond knives.

Acquire a stasis rifle.
You will need this to keep the Sea Satan frozen in place.
To spawn it, type this command:
ITEM STASISRIFLE 1

bam. Stasis Rifle. Now you are ready to take on the Satan of the Sea.

Listen to this song. Or intense, aggresive music of your choosing.

NOW.
IT IS TIME.
TO BEGIN.
THE KILLING OF THE SATAN

Now you must find your resident Sea Satan and SHOOT HIM RIGHT IN THE FACE WITH YOUR STASIS RIFLE!

Quickly now, swim over to him. Behind his face pincers and behind the front most flipper thingy.
Have both of your knives ready. Slash at him for about 15 seconds, then shoot him again with the Stasis Rifle at full charge. Rinse and repeat until he is dead.

And now
You have finally killed
the dreaded
SATAN OF THE SEA

but remember
he has brothers
and children
and they will be coming for you.

Your final result should look something like this:
Congrats, you just killed Sea Satan.