Digi Grenades are available from Level 1, right at the get-go. You can equip them on any of your classes in the armory by going to your classes' equipment section. You need to kill 10 people who are currently stunned by a Digi Grenade, meaning it must land and explode relatively close to them before you kill them. Something to take note of is that your secondary grenades can be pre-detonated by holding down the trigger before releasing. The only advice I have for this would be to throw lots of Digi Grenades before going for kills and try doing it in close quarters areas where the enemy is less likely to escape and be hit by the grenade or while they are hacking objectives. This achievement can be earned across multiple games.