The first mission that Colonel Rissler is available for use is level 08. You can also use him on the 'Fully Loaded' variant of any level, as early as level 01.
After you choose your game mode, scroll over to the right when selecting Command Team and choose Colonel Rissler. Then, just hold
during the level to fire his cannon. This cannon slows down all enemies hit for a short time.
Note on Colonel Rissler Availability
No AIs are available during mission 16.
As with all other achievements in this game, the achievement won't unlock until you beat the level.