Unlock everything at the last boss battle and killing him.
Before you put the last key in the hut (where you go to level to level) pause the game and go to the enter cheats. Enter BEWAREOBLIVIONISATHAND. Activate the unlimited Ammon, Don't Die, all weapons And unlimited lives. Unpause the game and put the key in.
Use the Firestrom Cannon and the Plasma Rifle to kill the boss. Use the Nuke or rocket launcher on the ships.
Ok, if you want all the weapon in the game you go to enter cheats and you put "oblivionsathand" make sure that it take up all the room then you will see the word "all cheats" that mean it all on. Then you will get the wepon then if you check the weapon you will have a secend weapon with it.