Ground is not that hard at all. I find the most common mistake people make is just pressing the 1 and 2 button. They might run two more abilities. In a massive encounter your tray should be all on cooldown. But don't be wasteful.
Next for survivability always carry Large Hypos. They are very cheap. Then get your Dodge bonus up high. You can do this with gear as well as the bonus from some set pieces. AND you can get it by crouching, sprinting, or rolling. Press C for crouch (+50% dodge), Shift = sprint (75% dodge), double tap W, A, S, D, Q, E for roll (+100% dodge). When you dodge a shot its damage is halved, BEFORE any resistance is calculated). So a 1000 damage shot turns into a 500 damage shot. Then your resistance is applied.
Flank Shots will get you killed. A flank shot reduces your damage resistance dramatically. So try to keep the borg infront of you. If you do decide to rush in, try to find a wall and put your back to it. It can save you from several flank shots.
I can solo an Elite Tactical Drone with my tac, sci, and engineer. I know my Sci and Engineer can solo Armek, I haven't tried with my tac, but I am very confident that I can. All borg will start slapping you when you are too close to them. This is great! Their slaps will put a smile on your face, since you know our mom hits a lot harder. Carry some Immunosupport Nanites incase they try to assimilate you. If they knock you back then great! That gives you several seconds of immunity to another knockback.
So if there is a large number of borg, hang back and crouch. Might as well press X for aim so you get the free 33% damage increase. If you face a single demi boss or boss, get close and let them slap you while you kill them. (Armek might be an exception, although with the right DPS giving him a facewave hug is a great setup).
Next try these kits depending on your profession:
Fire Team Kit
Starting out I would suggest you burn your kit as soon as you start to face the borg. This seems strange but most of your abilities will last several seconds and will carry you through the encounter. Include your Captain abilities. As you learn the benefits of each ability you can start stretching out your abilities. But dumping your kit will get you 70% of the way to doing the top DPS.