The absolute easiest way to do this is to start a solo zombies game on IX:
- First cut down a banner flag in the arena at the beginning
- Complete the first 3 challenges to get the Z-Harmony PaP Pistol as a gift from the Gods in the Challenge Podium.
- Visit all 4 towers, ring all 4 gongs, kill all 4 challengers, and feed their heads to the PaP Altar in the Temple area to open the Pack-A-Punch.
- Pack a Punch the Z-Harmony
until it receives the Brain Rot
ammo type down in the bottom right. There are 4 ammo types and there is some luck required. You should be able to PaP the gun at least 3+ times before round 9. If you don't have enough money to get one before Round 9, restart the process. The reason we use the Z-Harmony is because it will cost only $2,500 to PaP and get an ammo type instead of $7,500 ($5,000 + $2,500) for a normal weapon. You will have more chances at the ammo type we need for less money.
- With a PaP Z-Harmony
that has the Brain Rot
ammo type, begin Round 9 upstairs in the arena area if possible. Tigers will begin spawning from the cages, and shooting them can cause them to convert to a friendly tiger. Be careful that you dont kill them accidentally with a headshot, and the brain rot isn't guaranteed, it's still a random roll. Only tigers spawn in this round, so if you brain rot one, they will attack another tiger, killing them and gaining the achievement.