This type of mission consists of three steps:
1. Escort your fellow engineer alive to every of 3 weapon installations
2. Defend him while repairs are made
3. Escort him back to extraction point
The only difficult part is 2nd, as there are a hordes of enemies trying to kill your engineer for about half a minute. Simply deploy your sentry guns and droids (1 of each per repair) and make yourself a cup of coffee. Even on hard later missions you can afford that if you have your specials fully upgraded.
Do not forget to use gas and missle drones if you still need those achievements as there are large cluster of enemies here.
If you have any problems with difficulty at the later levels, just start a new game and make repairs there.