thats right, folks there is a plasma rifle that shoots scarab beams rapid fire. heres my tutorial how get it:

select level “metropolis” on any difficulty. once u get to the tunnel, leave one banshee alive with its driver. lure the banshee into the tunnel where the warthog comes so that his driver spawns.take the banshee through the level until you reach the point with 3 wraiths. now look around, there should be 4 tall buildings with bridges going from one another. fly to the left bridge. look for something like orange cones or something. U should see a floating plasma rifle. pick it up, but don’t shoot it by a wall or floor by you. and walla. thats the rifle have fun.