Cerberus has been imprisoned many times in the past, and hasn't been an official member of the Raven order for very long. He is currently working for a warlord group headed by the Raven known as Mumu. He's quite talented, but it's not clear why he chose to align himself with this particular warlord. Most think his best days are already past. He is encountered along with Mumu in the mission "Destroy Tansport Convoy."
Niflheim is a purple heavyweight armed with a laser rifle, handgun, and back mounted rocket launcher alongside an inside rocket part. With an emphasis for rocket launchers, the IO3RN-CORAL forces enemy generators to shut down from overheating before hammering away with the CR-WB85RPX. He is also equiped with a WR05L-SHADE as a main weapon. The unit has the tendency to suffer from overheating when using over boost.
For a tank, remaining mobile and pounding him with heavy weapons is completely viable. He is a heavyweight leg design so heavy cannons can easily rip him apart. Rockets are going to be powerful against you but if you remain mobile you'll do fine.
Missiles eat him alive due to a lack of a missile defense. Staying mobile will dodge the majority of attacks and you can typically get behind him, dealing severe damage to him while keeping you high in health. Note that he has an Optional Part that is capable of breaking the missle launch for long term missiles so relying on large missile volleys from something like a Small Missile Launcher could prove troubling for you.
Using the LX in conjunction with the dual KRSWs or the KRSW-OGRE combo also works and overheats Garm rather easily.
Like his partner Mumu, both are very vulnerable to overheating, a well placed overheating attack will leave him and Metis helpless against you.