With the help of teammates or explosives, you can force yourself into the vending/soda machines in the B2 breakroom (upstairs). From there, you can see outside the machine as if it were one-way glass, and shoot out. Furthermore, you can get pushed/explode yourself even further backwards into the wall separating the breakroom and the back rocks/water. This wall is thick enough to walk in, and you can roam around in the walls of building 2. From inside, you have a view of a large portion of B2, and can shoot and grenade these visible areas while remaining untouchable.
Detailed procedure: (works 80% of the time or so):
1. Wedge yourself in the middle of the two vending machines and face the other way (at the wall with the bench).
2. If you are alone, throw two c4 on the other wall and detonate simultaneously; if with a teammate, ask them to push you backwards into the machines.
3. Jump up and backwards, and you should end up inside the vending machine.
4. Use more C4 or have a teammate push you (still from outside the machine) to go into the wall.