Cerberus is a figment of imagination brought on by Dr. Tanaka's Noxious Gas. Cerberus takes the form of a large demon-like entity wrapped in a dark miasma. He is very fast and has a lethal attack. The apparition appears to terrorize Alpha Company after they Stop Dr. Tanaka from escaping Apollo.
Cerberus is one of the more annoying bosses in any campaign. Players will most likely be scattered when the fight commences. Be very clear where to rally, how to fight, and what to do to avoid his attacks. Cerberus appears after stopping Dr. Tanaka and at the tail end of the Noxious Gas Sequence. This means that players must still deal with the obnoxious third person camera. Regrouping should be first priority.
When Cerberus does appear, he will be somewhere in Apollo. Any players still returning from searching for Tanaka need to move with extreme haste, or stick to map edges to avoid Cerberus. The creature will make a B-Line for the largest cluster of marines, but will not hesitate to kill any marine or ally he encounters of his way. A Flamethrower, hopefully with Reactive Armor should stand ready to tank Cerberus. Any class standing up to Cerberus without Reactive Armor will be decimated in a matter of seconds. Classes with High Damage Skills should unload on Cerberus in an attempt to do maximum damage, possibly even killing the monster before he can enter his Nightmare Form.
If/When he does enter Nightmare Form, an audio cue and text prompt will appear. Non tanks should escape to higher ground or remain behind the tank at a safe distance. Cerberus will maul the closest thing to him, or anything he can see. If positioned properly this should be the tank, who should be prepared for the attack.
Always heal with Medkits during the battle. Should a Medic try to heal an ally, he will immediately draw attention from Cerberus's Hatred and will receive a brutal beating from the beast. If a Medic does make this mistake, try to stun Cerberus with either Monofilament, or Faith. Once Cerberus comes to, he will re-target the closest enemy.
When Cerberus finally dies, heal up and prepare to choose the fate of Dr. Tanaka.
The Marines will cry out warning each other to get out of Cerberus's sight. Cerberus will become invulnerable, then charge one of the characters in his field of vision and maul them mercilessly. His speed will triple to a total of 6. To make matters worse, his attack during this period can stun, effectively pinning the victim he is "chewing on." Make sure all players are a safe distance away and only tanking characters remain close to Cerberus. The only other real way to stop this is to Stun him then pray his victim can escape before the beating continues. Likewise, forming makeshift barricades out of expendable units like Disruption Towers, Field Probes, and mind controlled units can help to physically keep him from reaching his target.
Cerberus has an extreme hatred for Medics. Any Medic who attempts to heal himself or allies will instantly draw Cerberus's full fury. There will be no escape, as he can outrun medics easily. The only hope is to Stun Cerberus, and then have the tank reacquire his agro. During the battle all players should heal only using Medkits, Bandages, and self healing skills and items. However Nano-Shield will not trigger his hatred because it does not heal.