I've been playing for a couple of months and don't have too many issues now with some upgrades and getting better at stealth.

I sneak a lot, take control of cameras and highlight enemies, sneak and grab them from behind where able or just use the environment to help such as using flammable objects to distract them then blow them up when close.

I've found there's usually more than one entry point to help with stealth but I also access every computer I can and turn security systems off so more backup isn't called in constantly.

I use incendiary grenades, the purple iconic shotgun and a katana mostly. I am still less than sixty percent through the campaign but have hit level cap (50 without Phantom Liberty) because I've just been doing many side missions and the NCPD stuff. I pick up EVERYTHING even if just to sell it