Assault Trooper is a basic, common, and cheap offensive strategy (or simply put, a role) that works best at mid- to close-range. Essentially, it is available in MiniTroopers right from the start. All you need to do is give a Trooper a gun with a suitable range, and there you go. Assault troopers are common grunts you deploy en masse, hoping they overrun the enemy.
They can also be considered a default state, from which you can upgrade them to other Roles. As such, while initially remotely effective, with the proper upgrades, they can achieve great results even without switching to a specialized strategy.