First go to weapons and right click the weapon you want to pick for the npc.Then click on copy to clipboard and then go to the spawnmenu and click the weapon selection and delete the weapon currently selected and copy and paste it (ctrl+v[in order]).

here are examples:
1) selected npcs with nukes and they fire at you and kill you
2)uses the most powerful weapon in the world
3)uses nyan cat
4) some of the doom 3 weapons
5)half life 1 weapons
6)etc

Note:some of the weapons wont work