This applies for the first 3 campaigns and less for Ada's, also this is for people playing through on professional the first time. This is also more targeted for people playing through solo and not co-op.
The most important being field medic, this allows your AI partner to heal you at lvl 1 giving you 1 health tablet (sometimes 2 but rare), while this won't keep you from going down quick again 1 health bar will keep you from instantly dying when you do. This saved me a lot of headaches and your partner is pretty good about getting to you.
The next is firepower, when you max out this skill and add it with juavo killer/zombie you will slice through enemies even on professional. While normal enemies aren't the larges pain this will allow you to save a TON of ammo so you always have it when you need it.
Don't be scared to sprint past enemies, and go for the door! There are a lot of parts that are easier if you just sprint past enemies and get to that door, I found myself using this tactics more and more as I played through
Finally my biggest recommendation for the game is when you are starting your game select to play offline. I didnt want a partner anyways because it takes away field medic (works with AI only), but this will let you pause your game and avoid the pain of sitting through a 1:30 chapter.
Hopefully these skill tips help people some people.