Flood tank form/Gameplay
From Halopedia, the Halo wiki
The best way to take a Tank Form down is by simply shooting it in its head or mouth before it can spit Infection Forms, though once hit it will often cover with one arm for protection. If you hit its mouth with a projectile weapon (ex. Battle Rifle) it will count the shot as a head shot. All Flood Combat Forms have this supposed "head mouth". If you want to kill it with one hit, since normal melee attacks usually take more than one, use the Energy Sword or Gravity Hammer. Or just throw a grenade that can stick to it, like a Spike Grenade, Fire Bomb Grenade, or Plasma Grenade and it will kill it instantly, or you can just simply melee it with a Brute Shot. A melee attack with a Brute Spiker also seems to be effective although it may require more than one strike.
They are extremely tough, they are the Flood's equivalent of a Hunter, and attacks that destroy normal Combat forms with one hit deal far less damage to a Tank Form. They are able to take multiple hits from the Gravity Hammer (only takes one hit if a player hits it directly between the two tentacles hanging from its face) and the Shotgun, though the Energy Sword seems to make shorter work of them, as will a melee attack in the mouth, usually killing them in one slice. If you shoot it and it crouches, posing as if it were dead, it will come back up when you are close, this rarely happens but make sure it is dead.
Since they are very slow and unable to rush the player, the Flamethrower, or Firebombs are a simple, ranged, one-hit option attack against the Tank Forms, though rare. Beware, they can move on all fours, increasing their movement speed drastically. Brute Shots are also extremely effective against them because the melee attack with the Brute Shot is an instant kill against them on all difficulties except Legendary, which takes one shot with one melee. Plasma Grenades, are also very effective, but only if they are placed near the head area. A pair of Shotgun blasts to a Tank Form's head is sufficient to put it down on Normal difficulty.
There is a more unconventional method of jumping onto one's head and either meleeing them or using a powerful weapon, as they are unable to attack above them.
When you see a Tank form you must remember these hints: