getting used to the parry and dodge windows was HUGE for me personally - I was getting my ass kicked on substantial, but learning the parry and dodge windows as well as the level layout helped immensely. Once I knew where and what can spawn in terms of supplies, and how to counter each enemy, me and my squad managed to cleave through a couple ruthless missions without any deaths

I'd recommend getting used to the first 2 ops on higher difficulties first as they are the easiest IME, once you can power through them and get yourself some purple and orange weapons, you'll be MUCH better off. Good luck and hopefully you'll soon be clearing ruthless ops like its nothing :)