I went for the rocket launcher before I went up the elevator but the most
important thing was to stock up on health packs. I put them in the room next
to the elevator so I could run over them on the way back to the office
console. Another thing, duck down behind the console on the catwalk, collect
the goodies, pull the lever and run like hell back to the elevator. Take out
a couple of combine waiting for you outside the elevator room downstairs and
from there it was just run and gun. Don't try to take out all of the
combine...I don't think you can. It isn't that far back to the office and
once you're past that first set of stairs you're protected by the wall
almost all the way to the steps at the office. Tip: before you trip the
office console, quick save. Sometimes there's a bug at the ramp upstairs
when you run after Breen. Going through that fight once is enough.

I guess you haven't played HL2? The energy balls are an integral part of the
end game.

HTH
Randy