It's important to be aligned in your goals - there are objectives all over the map and whilst nipping off to clear out a small nest sounds like a good idea, it can quickly escalate and mean the rest of the team need to come and bail you out, which also rarely goes to plan. Stick together as a group and communicate about where you are heading and who is doing what.
Once you have that down, you can probably survive long enough to extract.
Now, to optimise. Be sure to use your dive A LOT - it's the best tool for getting out of a tight spot quickly.
Don't save your stims, you can resupply often and usually find some restocks along the way, use all the stims all of the time. Same with your grenades. You can't take these with you when you die and when you're health is less than 80%, there's a big chance that a big hit is taking you out.
Clear waves before pressing on. Usually you can run into multiple waves or roaming groups by trying to escape - it's usually safer to stay and clear the current wave before moving. Or else you will get overrun.
Also - just enjoy yourself, it's a bloody brilliant experience with friends.