Discoid is the move that puts a purple reticule over someone and bombards them with orange orbs. But I believe the tank buster actually occurs during Discoid. I think the tank buster is called Perpetual Ray after looking over a log for a random DF run this night. Appears to happen 4 times in succession:
http://i.imgur.com/az9rqTj.png
Hydrothermal appears to be the 5th 'hit' but occurs around 4 seconds after the Perpetual Ray barrage. It could just be move in his rotation that he sometimes throws in during the Perpetual Ray combo, though. There's a fairly large variance on its damage, too.
Can any tanks verify what order this occurs in/when? Would be nice to get a supervirus off beforehand.