Manually detonate a grenade in the mid-air. Your transparent outline will be the only thing visible until you you fire a weapon. Complete round 11 to view a cinematic that shows a large number of zombies overrunning the area. It will infect your enemies and turn them into zombies for round 11.
Complete round 10 in Riot and a DNA Bomb will drop. To unlock Tier 4, complete 100 rounds in the Tier 3 maps. To unlock Tier 3, complete 75 rounds in the Tier 2 maps. Tier 1 is available by default, and you must complete 50 rounds in any combination in those maps to unlock Tier 2. To do so, you must unlock each tier in succession. Complete the following achievements to unlock Microsoft Gamerscore points.