Under the Forge tab in the main menu select 'Build' and load up any map. Once in game press 'O' to open the object editor and select Weapons at the top of the list. With the weapons tab open select Power Level and then select Tier 1. Under Tier 1 you can find the Halo 1 Magnum and the Halo 2 Battle Rifle. Once you've spawned them both select the weapons tab again and then select Tier 3 where you can locate the SPNKR Rocket Launcher. Once they're all spawned in the achievement should pop.
Video guide for those who prefer: