A nice mix of feedback and advice in this thread so far. I know the Roosevelt Island mission can be a real endurance test, especially towards the end. It's one where good or bad positioning and movement can determine the flow of the fight at any moment.
My best advice is - and definitely when playing with others - to make sure that your builds, playstyles, and tactics mesh well. A player who is built for CQC and pushing forward does not have good synergy with a a squad built for long range poking and picking off. All that leaves you with is a dead ally and enemies prepped for four of you. The mission is definitely hard, it is supposed to stand as a barrier for even harder content in the subsequent world tier. But it is definitely doable.
Hope this helps.