This achievement, along with all online achievements for this game, is now unobtainable. The multiplayer servers have officially been truned off as of August 11, 2011
This one may sound hard due to that fact that you must complete 5 repairs without dying, but it's really not all that hard.
First you need to spawn in with the Engineer kit, the only kit with the torch. Then you'll need to find 1 other person on your team that is willing to talk and work together. Then the 2 of you hop into a Tank together. One of you will be driving and operating the main cannon while the other is is positioned in the gunner's seat using the machine gun mounted on top of the tank. Just traverse the map taking out anybody that you see. Always be on the look out for mines and destroy them as soon as possible, it only takes one to ruin your day. People love to put these things in shadows, make sure you double check them. Also, Snipers have a laser designator that will take you out in one hit. The laser designator's weakness is that it drops from the sky, so it's always a good idea to have shelter above your head. Garages and bridges are going to be your friend. Eventually you'll take on damage as the enemy tries to take you guys out. Once the tank's armor is at 50% or lower the gunner should hop out and repair while the driver continues to eliminate threats. You might die quite a bit at first but eventually you'll both learn your role and you'll obtain this with ease.
After you reach the required 5 repairs you'll hear a "Buh-dum" sound followed by a message that will pop up on your screen notifying you that you've be awarded the Expert Mechanic Medal. No achievement quite yet, just keep playing until the round is over. To get the achievement to pop simply join up another game and it should pop once you have loaded into the match. I personally had problems getting the achievement to pop if I left or got disconnected from the match where I had met the requirements. If that happens you will need to go through the steps again.
Just to make it clear, repairing a vehicle that was damaged by friendly fire does not count towards the achievement. Also, a friendly player must be in the vehicle while you are repairing it.