The whole point of moving from left to right is that you DON'T get surrounded. You engage the flanking squad at the chopper, move to the base of the tree to engage the main wave, then move back and forth as targets appear. The idea is to kill these waves fast enough that you never get surrounded, which is difficult but not impossible on Hard. You solo the first four waves, which only have Goblins. You buy the MP-40, as Orcs will start spawning wave 5, and it should hold until you can get something with more bite from the gun shop, which will hold until you can get the upgrade drugs, which will carry you to victory.
I'm not saying this is the only viable strategy, nor am I even saying it's the best. I'm saying it's one that I've found to yield consistent, safe results, and is my preferred way to run the map.