The Tanket is a special enemy in Final Fantasy X, created in the Species Conquest at Monster Arena when three of each Helm-species fiends have been captured, with the exception of the Swamp Mafdet as that is only encountered when capturing fiends is not possible.
Like its fellow Helm monsters, Tanket is encased in a harder-than-steel shell, meaning weapons without the Piercing ability will do little damage. Although it may appear slow, Tanket has a couple of nasty tricks. His normal attack inflicts Berserk and Delay (strong) on its victim. If only one party member is alive, Tanket will attack them continuously without giving the player a turn until they have been defeated.
The best strategy is to take it down with party member's with fully-powered Celestial Weapons before Tanket gets a turn. A weakened Auron with the Masamune is advised for maximum damage, preferably alongside allies of: Rikku with Godhand, Kimahri with Spirit Lance, Wakka with World Champion, or Tidus with Caladbolg, both at full HP. Tanket has a relatively low Overkill requirement, so scoring an Overkill should be achieved by any player with the Break Damage Limit, assuming their Strength isn't lacking.
Tanket is not immune to Mental Break, but it will only work 1% of the time. Also, Tanket is not immune to Armor Break, but it will only work 10% of the time. A Frag Grenade or Auron's Banishing Blade Overdrive work 100% of the time, and so are more effective to break its Full Break resistance.