You have to use the card on the projectile itself, not the enemy, so the projectile can bounce back towards the enemy.
If you were on level 3 (the level where the first type of enemy gets introduced), that's why you didn't see more of the slow down. The next levels heavily use the slow down/stop time - you can easily handle incoming bullets.
If I would change something in the level design, it would be to directly show the players that the enemies can kill themselves with their own bullets. Thanks a lot for thought-provoking feedback.