Also, get a sniper silencer, and pick off first the alarms, and then anybody who's way out of the way and won't be noticed for a while. And if I recall correctly you only lose the stealth bonus if you're actually spotted, so you can kill enemies in sight of others, although of course this is risky - however, it can work well for instance if you lob a grenade over a fence into a bunch of guards that you've previously tagged, then run away and make use of the quick crouch steath skill, and pick off the remaining guards while hidden, moving location regularly.
Disclaimer: I played on easy, so these strategies may just be nonsense on higher levels, but I feel I'd have a good chance with them.