This is one of the game's easier boss fights, and you get the achievement when you beat it.
To begin with Cerberus has three heads. Notice also there are four light coloured patches on the ground (two near you and two close to Cerberus). Cerberus really only has two attacks - spitting stuff at you, which can be easily evaded either by running or using the RS, and lunging at you with one of its heads.
Basically, you need to destroy one of the coloured patches (RB and then mash B), and draw one of Cerberus' heads on to the resulting fire.
The easiest way I found to do this is to stand still for a bit near one of the rear patches, wait for Cerberus to lunge, evade, hack away for a bit, and then destroy the patch as Cerberus is pulling its head back. If you try and destroy the patch while the head is up, you probably won't be able to evade Cerberus' lunge in time.
Anyway, once the fire is burning, force Cerberus to lunge at you into the fire. This will allow you to slash away happily for a bit at one of its heads.
It is much safer to use the patches at the rear of the screen as Cerberus will use a different lunge attack when you get close which is quite tough to dodge. The patches will respawn after a little bit too so I found it best just to wait for these to reappear rather than trying to use the front ones.
Do this a few times and you will eventually be prompted with an "RT", this will initiate a QTE which destroys one of its heads. You need to then repeat this for the other side. This leaves you with just the middle head, which can be beaten with exactly the same strategy.