Any explosive weapon or grenades work.
I.e. Frag/Plasma Grenade, Grenade Launcher, Concussion Launcher, Fuel Rod Gun, Rocket Launcher, Artillery Strike Target Designator, Needler, Needler Rifle, Plasma Launcher.
If you damage it's armour, it's back becomes more vulnerable. Using weapons suck as the Assault Rifle, Shotgun, Spiker, Plasma Rifle and Plasma Repeater will do lots of damage.
Have allies distract them, or you can give your allies explosives with low ammo and have them do some damage. Invulnerable Allies such as Noble Team members are also key as they cannot be killed. They make the perfect distraction.
Bad Derek, Marines are useful for your low ammo explosives, so keep 'em alive.