It's probably quicker to do it manually. It'll take you less time than you think. 300 units at 3s/unit is only 15 minutes.
You could create an array of all units on the map, detect what weapons and ammo each unit had and remove some, but by the time you've figured it all out and tested it you might as well do it the easy way. The easy way also helps with lag, which in a mission this big could be an issue. Never use a script when the mission editor will do.