Tanker is a boss in Final Fantasy X. During Sin's attack on Dream Zanarkand at the start of the game, Tidus and Auron will engage in three battles. The final one will be against the Tanker and Sinscales.
It is impossible to defeat it with Overkill.
The Tanker spawns on Auron's 2nd or Tidus's 4th turn. If Tidus gets sufficiently more turns than Auron, the Tanker will spawn with Tidus saying '(Hmm...)'. 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.
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.