I haven't managed to kill a Ruvik clone with stealth. If you creep up behind them and get as close as you can, they will turn and see you - well, this is what's happened every time I've tried. Point blank him with a shotgun blast, it should only take one or two.
I've reached chapter 11 and I think I may have been focusing on upgrading the wrong weapons, that is handgun and shotgun. I think it would be better to upgrade the various crossbow bolts. An example is the flash bolt. Upgrading this means enemies will stay stunned for 10 seconds, in which time you can stealth kill a few of them - there is less ammo drops in this difficulty, so stealth is the way to go. Also, I'm thinking the normal bolt fully upgraded sets enemies on fire. Very useful for the Laura fight (both of them!) and bosses.
Maybe it was obvious to other people, but the more I play this game I'm thinking it's less about guns, more about the crossbow. Also upgrade some stamina.
I don't know how people have managed Akumu. Chapter 6 and 10 on Nightmare difficulty are some of the hardest parts of any game I've ever played! Though it's worth it as I think Nightmare mode on newgame + will be an enjoyable difficulty.