Twice now I've played Cure Ground Norm. and both times everything was found up till the boss. Armek is bugged, and overpowered! We had a great team, and we can do the other ground missions without a problem. So something is really up. Armeks attacks are more like they are set for an Elite. To add to this, we have never, ever, gotten any injuries from any normal STF, except from Armek!
1) You shouldn't get injuries on Normal. That's definitely a bug. You sure you didn't pick Elite by mistake?
2) Armek has fairly absurd damage output, so you have to use specific tactics, even on Normal difficulty.
Here's what I do:
Put a Science Officer armed with a Medic Kit and a melee weapon right in front of him. That person should keep hitting him in the face and use their medic kit to keep themselves alive (Triage and Vascular Regenerator have damage resists, so they should be popped any time their up). Because that player is at melee range, they'll hold Armek's aggro.
The other four players should stand right behind him (you need to set this up before he activates, so wait outside his 'arena' until everyone's ready and then all run in and get into position whilst he's talking) at about 2.5 metres distance armed with Pulsewave Assault rifles and continuously shoot him in the back. It's important to be directly behind him, not a little to the left or right, or else he'll be able to use his chain/AoE weapon to damage the entire team. If you do this right, he'll only be able to use physical attacks and his assimilation nanites against the Medic and won't touch the rest of the team (you might get caught with an Orbital strike, but one of those shouldn't kill you). The tricky part is when he spawns his Covershields because you can be caught outside them if you're too far away, and have to move around them giving him a chance to chain. It might take a couple of attempts to get right if you haven't done it before.
Don't use any pets like Security Officers, Drones, Hortas etc because they won't stay in the right position, and will give him targets to chain. Grenades, Mortars, Turrets and Transphasic Bombs also mess up his aggro: he'll go after the turret or whoever used the grenade or bomb instead of the Medic. So don't use those either.
If you haven't got a Medic or struggle with this tactic, there is a messier version whereby everyone just stands in a small circle around him with Pulsewaves and shoots him until he's dead. This will result in people dying because he can use his chain attack and you're sort of relying on lucking it out (which you usually will eventually, might take a few attempts though), so I much prefer doing it properly and using the first method I suggested. Cos once you get used to the first method you can reliably kill him without dying.
If you've been doing one of the above methods and still aren't scratching him, then I'm not sure what to say.