Yeah. Big mistake.
Grav-cannon range is 24" and Salvo 3/5, so with the Centurion SnP it's 5 shots all the way even if he moves them. Which he doesn't. 24" means that as soon as you close into bolter range, they have you.
I beg to differ. 24" means that
IF you close into bolter range, they have you.
Don't try to beat them at their own game. If it works, it's going to be costly. If you insist, a cheap solution might be to drop two (if not three) Ironclads in on turn one. Grav can destroy vehicles, but only en masse. Split fire will not be enough to take out both. That leaves the Tacticals with plasma that is as much of a stretch. Sure, you're most likely going to lose one, but the other one will either take care of the Tactical screen or, in the best case, be able to get into close combat with the Centurions, which should solve that problem. Give them twin heavy flamers and you might even take a bloody toll on the Tacticals the moment you come down.
But the main problem for a formation like that is that once the Aegis is set up, you know where the Marines are going to be and going to stay. The clever thing to do is to bring weapons with longer range and just let the Centurions sit there.
Snag is, of course, that I might be able to out maneuver the Centurions, but then out comes Khan and his Scars to take me in the flank....
Snag? Centurions and Tactials behind an Aegis, that's half of a 1000pts army. When Khan arrives (does he have a rule that lets all of his boys arrive on the same flank? If not, even better), you will have twice as many points as he does. Yes, he gets to shoot first, but after that it's still an uneven fight, in your favor.
Here's the question. Do you play missions? In some missions this is easier than in others. If you can freely distribute half a dozen objectives, it'll be harder because there are bound to be some within Centurion range. But in the kill point mission, there is no incentive to go there in the first place and you can win by taking on the Scars, or, if he cowardly holds them back, use your superior range.
Hey. Here's another creative one. Assault Aegis. Bring your own Aegis, set it up like this:
..................________................... Enemy Aegis/end of deployment zone
.................................................. Middle of the board
/
/
/
..................._____/...................... Your Aegis/end of deployment zone
Well, not so steep. A 45° angle should do it. Advance behind the Aegis. The unit that gets shot by the Centurions goes to ground. They'll sit there for the next turn taking pot shots, but your other units, can operate normally.
If you set up your Aegis first, a vertical approach right across the center of the board will be best.
................................................. Enemy end of deployment zone
......................____..................... Middle of the board/end of your Aegis
[ ]
[ ]
[ ]
......................[ ].................... End of your deployment zone
You can deploy close to it and funnel into it, or infiltrate Scouts into it from the beginning. What do you care if their armor is worse if you get a good cover save? 3+ normally with cloaks, 2+ if you go to ground like everybody else. Use the long sections for the sides and the short ones for the front. Should give you a 6" broad corridor right up to the middle of the board.
The best thing about this is that it costs you a mere 50pts. Since it centers around taking the center of the board, you also get an extra turn of respite from the White Scars. If you push the enemy Aegis into a corner, your opponent gives up board control and lets you play out the range advantage that I still recommend as your best option even more. If the White Scars deploy on the board, you may just give up your Aegis completely and deploy at range yourself.
But again, if you're set on beating the Centurions at their own game, this can also be combined with the dropping Ironclads. If you want to get extra cheecky, you can combine this with naked Scouts in Storms. Maybe a power axe for the Sergeant if you hope to make it into close combat with the Centurions. Flanking has to be announced during deployment, so if the White Scars flank, you are guaranteed two turns in which you can take out the Centurions and Tacticals undisturbed. If you commit your entire army, you can actually get this done, If he doesn't take out the Ironclads, they'll rip his line to shreds. If he does, who's going to take care of the Scouts? And then there's the rest of your army advancing behind your Aegis, potentially Terminators with good fire power to inflict extra casualties that can go to ground and basically have a 2+ save against anything that wants to take them out.
The alternative is a trukkload of Burna Boyz wiv real burnaz - build a cigarette lighter refill full of butane and a computer cooling fan into the chassis, two into one the feeds to the front of the truck, a piezo sparka and teach 'em da burny dance (I love da smell of toasted polystyrene in da morning 
)
