Beach Party is the ideal mission, which has lots of Medics, reinforcements and enemies.
Once your boat lands on the beach, fight your way to the bunker wall. Once you ascend to the level of the machine gun nests, you'll start getting a steady flow of Medic and Engineer reinforcements, so you'll have more Medics if you die.
The easiest way to get this is just by healing yourself. Just move around the bunker very slowly, trying to engage only 1-2 enemies at a time. What you want to do is let the enemy take the first shot and wound you, but then IMMEDIATELY kill him and run back for cover...you have to be quick, otherwise they can drop you pretty fast with their automatic weapons. Heal yourself and then slowly approach again, using the same method. If your Medic dies, just switch to one of the reinforcements and try again.
One of the tanks on top of the hill may be giving you trouble as you pass by the open bend of the bunker, so what I recommend is that you switch to an Engineer and hit it with one rocket. The tank will start to move around and won't bother you as much.
Remember, while you'll unlock the achievement immediately, make sure you DON'T QUIT once it unlocks; either finish the mission or just fail...if you quit, it won't save the medal towards your campaign progression!