Multiplayer is insanely fun, trying to get my fiance to play it. I've already gotten as good as I can at MP, so I haven't played in a year.
Setting up traps/ambushes is so great. The Enforcer w/ Silencer and Covert 4 (iirc) + Fast Crouch/Silent Crouch = a lot of kills. Tactical shotgun is satisfying, but a little OP.
About Grounded mode...
Yeah, that's what I came to after finishing Survivor. Don't know if I'm good like that, but I had no issues; fuck getting hit... dodging and punch combos/finishers and bricks for clickers is all you need to beat the game. It's not difficult at all, just involves patience.
Seriously, using a brick on a clicker (or anyone) is pretty badass. Overall, you can beat this game without using anything other than what you find (using shivs is pretty useful, but you can do it without them, too).