ehmm....if i can remember right the engineer had the option of dropping supply packs, and you can drop them for yoursellf. instant re-supplying you with ammo and health.
just stick with that and you'll be fine. atleast that worked for me.
and once almost out or out of those packs just re-supply yourself at ammo machine.
and it helps if you have the special gun, the 1 your awarded with after taking out a certain amount of enemies with the standard weapon.