After you get a sizable honor guard, regular stages are simply easy, even when they attack with, like, a size 14 army where you have to take out two bases. As the Space Marines, I got the terminator unit rather early and he alone could take out enemy bases, I swear. As Chaos, I got myself the Possessed Marine and Khorne Berserker champions and they just ate things for breakfast. Ork honor guard wasn't that impressive, I admit, but then again, they were actual squads so if you lost any units, you could reinforce them, increasing their longevity at the very least.
I want to do Imperial Guard next because I absolutely blow ass with them as a race. Their honor guard is terrible, though. It's literally just a regular squad of guys.
At least they can get a Vindicare Assassin in their HG; that should help greatly.