During the battle, Auron will mention to Tidus they should focus on attacking the Tanker. Unleashing a few strikes upon it and ignoring the Sinscales will end the battle quickly.
Alternately, if Tidus gets sufficiently more turns than Auron, the Tanker will spawn with Tidus saying '(Hmm...)'.
The Tanker spawns on Auron's 2nd or Tidus' 4th turn. If the Tanker spawns from Auron's cutscene, the required hits is (8 - (number of player turns before the Tanker spawned)). The amount of hits required thus vary from 4 to 6. If the Tanker is spawned by Tidus' dialogue, it takes 3 hits to kill, with no regard to turns.