I am really sorry to torture you like this! I should probably be a bit more precise in my advice: first, boulders are not the only possible projectiles (see level 6), then you should find a way to "reload" another ammunition if your first shot is not accurate enough (do not forget to *reset* your "weapon" before that...)
PS. Maybe you lose time in the first part of the level. Knowing what to do, It takes me about 2 minutes to get an agent near the steam.
I still thank you very much in return for your "tons of thanks" and your indulgence. If you know other fans of logic systems and complex machinery, do not hesitate to lure them into this trap(!): collective intelligence is probably the best way of reaching level 72... even if I'd be very happy to help here and there ;^)