You need the truck invincibility glitch, use the truck with the barrels beside it's ladder, because it's so close to the Weapons Armoury, so you'll re-arm every two-four rounds, after using the multiple weapons glitch of course, and make sure that you have the enough ammo for the next round, because if you don't have the enough ammo, well, you have nothing to do about it... I highly recommend to use the multiple weapons glitch that depends on one Sentry Gun and a ladder, because it conserves some money for your weapons and exploits the ladder of the truck's container even more, however, becareful not to step out of the container, because it very narrow and -sometimes- it become impossibe to go back in the container.

Here are the videos:

1-The truck invincibilty glitch:



2-The multiple weapons glitch:



Note: These videos are NOT mine, I just found them in YouTube, tryed them, and liked to share them by adding some tips.