Um, whirlsand is only used when you have two or more characters alive, and the tentacles are not put down into the sand. If you enter the battle with only one character alive, it'll stab the tentacles into the sand and you can then revive them both without having to worry about whirlsand.
However, if you kill the tentacles later, it will use whirlsand again.

Also, he is only indesctructible while his tentacles are not in the sand.