step 1: start the level on normal difficulty from rally point alpha.

step 2: the level goes on normally until you reach the first structure

step 3: kill a grunt to get a plasma pistol

step 4: overload the plasma pistol on the falcon dropping the warthog, so that the falcon will crash on the ground

step 5: get in! WARNING: once you get in, you won't be able to get out unless you restart!!!

hope it helps!