1 Laser each floor will get you very far (supplement with machine guns as necessary if you cant afford power or another laser). After that try adding a power blast, slows them and since your laser hits everything on the floor more time makes life even easier. Add more lasers as needed don't upgrade till your out of weapon slots. You'll win but be bored. The rocket is a major waste.
Also I'ld recommend adding in some red herrings. I figured out allot of the levels by looking at what pieces I had left and how I could combine them. Some extra bots would realy confuse people.