theres 2 endings one where u face the final boss and one where u dont.

G5 pistal - 9th or 10th level
(Fast fire rate and good damage but not that good compared to the other weapons)

Nuclear launching bazooka - last level's "shop"
(Really low rate of fire and not that effective aside from large masses of small critters)

Auto shootgun 200 something - level 11 if I remember correct, light version at last level
(Tested against all other weapons on the queens and last boss - most powerfull gun)

Vulcan minigun - last level's "shop"
(Fastest rate of fire of all guns, extremely effective but needs alot of ammo)

Ion Launcher - level 10 or 11
(Most single shot damage of all the weapons aside from the nuker but slow rate of fire)

3rd heavy power armor - around level 11 or 12
(99% damage reduction but due to the amount of enemies I needed quite a few of these)

Used the nuke against the wave of enemies that appear at first and then switched to the chaingun to hold them off for as long as possible followed up by the shootgun when they eventually got closer and finished of the remaining ones with the Ion gun, rarely used the pistol aside from smaller groups of surviors.
(Last level's boss room were quite insane, damaged all my armor's and droids and nearly ran out of ammo for the weapons)