You can activate Elusive Tormentor's ability multiple times to discard multiple cards. Only the first instance of the ability to resolve will cause it to transform.

Once blockers have been chosen, transforming Elusive Tormentor into Insidious Mist won't cause it to become unblocked.

You can, all within one turn, attack with Elusive Tormentor, transform it into Insidious Mist before blockers are chosen, transform it back as its triggered ability resolves, and have it deal 4 combat damage.