I got this from a New Game + on Normal difficulty, with a maxed out plasma cutter.
you can have other guns in your inventory, and even upgrade them towards your "maxed out" achievement, but don't fire them.
It's wise however to drop all weapons apart from the cutter into storage, as the game will give you more plasma energy from enemy drops, which is the only ammo you want. if you do happen to pick up other ammo, sell it, and buy more plasma ammo, or nodes to upgrade stuff you haven't already.
*NOTE* using the ships defence cannons do not disable this achievement.