I don't know this for sure, but I suspect the difficulty of level 4-L is intentional. In a lot of FTP games, they sprinkle levels like this every once in a while as a way to slow you down.
My advice is this: I beat the level with a team of level 43 characters and I picked Old Ben as an ally. His special ability gives the storm troopers ability lock, which prevents them from throwing the grenades. This gives you an extra round to kill 1 or 2 storm troopers before they unleash the barrage, which hopefully makes it survivable. The cooldown on this ability is quite long, so I couldn't use it in the second round, but the storm troopers in round 2 start with empty turn meters, which helps. Use the ability again at the beginning of round 3. Boba Fett isn't that hard to kill if you can get to him.