I think there's something about the ground mechanic that I'm just not getting.
I was doing a basic exploration mission against Rommies and I kept dying on the boss pack. After about five tries I just gave up.
After the third try, I left and spent every EC and DL I had on better gear for my BOffs and it didn't seem to help any.
I tried everything I could think of, CCs, kiting, using all my buff/def cooldowns, but I just kept getting overwhelmed by the Engineers and all their gadgets. Even when using BOff focus fire on my target, I couldn't seem to burn them down fast enough because the Medics would heal them. If I went after the medics first, the engineers would get up their gadgets and overwhelm me.
The CCs just don't seem to last long enough to be very useful. Which is strange because the ship CCs are excellent.
Against non-borg enemies, I target enemy Engineers first (and tell my away team to hit my target), then Science officers and Tactical Officers last. I also hit the Cmdr Engrs first and work my way down the ranks. A high level Engineer can ruin your day with all toys they can deploy. If for some reason they have a high number of Science officers or medics, hit them first to prevent them from keeping thier buddies alive. This is my strategy and it works well for me; however, your actual mileage may vary.
You may want to check if you're playing on Standard level. However, you shouldn't encounter too much difficulty even on Advanced. My daughter can even do some missions on Advanced (I helped her build her away teams).
I'm not an Elite player; but my characters progress through missions without any problems. If you tell me more details, like what char type, I'll give you suggestions on away team composition and equipment loads. Forgive me for using the term, but I strive for a synergy when building the away team. So instead of deploying 4 crack specialists each doing his own thing, I try to deploy a crack team of 4 specialists that work well together.
Added after reading Intrinsical's post.
I didn't really get into tactics above, but one of Doc Lee's indoor tactics is to assign one BO to stay put, one on the left side and one on the right, (and one free), then run through the crowd to the back side, turn around and blast them. So him and his team are hitting the enemy on four sides.
You can also try something simple like crouch (c) and aim (x).
Outside, I look for key terrain, that is high ground especially a cliff. An enemy will eventually jump enough to scale a cliff, but he'll be hit by enough weapons fire with kickback that his silly ass will be back on the ground before he knows what hit him (and possibly dead from the gunfire and falling damage). This is a good tactic to pick off enemies. Plant your BO's on a cliff (out of sight of the enemy), then move your char to the cliff edge, take a pot shot and then back up and wait for the enemy.
On some missions, killing the boss will vaporize any remaining henchmen. So concentrate all firepower on him. If you have a character with Seduce, use that on the boss so he'll kill his own men. Hit the boss hard before you engage with your team. Things like mortars, orbital strike and plasma grenades work well.
There's a lot of different tactics to try. You have to find the ones that work for your team.