When the enemy is above you on the platform, you really don't have any angles to shoot at him. People don't usually expect an enemy below them to be able to do any damage to them, so these two following techniques work especially well:
 
1. Shooting rockets at the wall
 
When you're below, shoot rockets at the wall, up at an angle a hair above the actual platform. By doing this, you're using the rocket's blast a.k.a. shrap to hurt the enemy rather than the rocket itself. If the enemy is running towards the RA, you can shrap him several times before he picks up the RA.
 


Shoot the Wall!



 
2. Rebounding grenades off the wall to land on the platform
 
This technique is a little different than shooting rockets at the wall. Grenades often don't explode immediately (unless they hit someone directly), so this is a tad more on the defensive side than the other. As you're running underneath the platform, bounce the grenades off the wall, aiming up at an angle of about 45-70 degrees, depending on the distance you are away from the wall.
 


Rebound Grenades

Next: On the RA Platform