As noted, firstly they have to be standing outside a land raider - in fact, they can't be in any transport and do it; all the marine transports which do have fire points explicitely forbid Bulky and above models from using them.
Also, whilst GaleRazorwind's advice is good, in the specific case of Centurions, you don't have to deal with overwatch, either; slow and purposeful means you cannot snap fire with them in response to being charged.
Grav Cannon centurions are good - two or three grav cannons can realistically butcher a monstrous creature - even a big one - in a single turn of fire. But they are short (ish) range - to be effective, he's going to have to put them in or near his front line, not back with the rest of the heavy support. Fling raveners, hormaguants, that sort of thing at him. Hormagaunts for preference, just to let him flail uselessly with the grav cannons at something with a 6+ save. A big brood might kill them (especially with toxin sacs) but ultimately, think gribbly - if you trade a hormagaunt brood for those things not firing for the rest of the game even if they're just locked in combat and not dying, you've still come out on top...