Before you finish and exit the end of the second mission you will find to the left of the building a snipers nest, from which you can fall a small distance and damage yourself. Fall off, damage yourself (you heal a little bit after) run around,climb up and repeat. If you suffer too much damage, just use a med kit, then continue.
Do this about 35 / 40 times and then complete the mission.
That should do it.