(Achievable in either Heroic Frog mode or Adventure mode - Adventure mode is obviously easier)
Get to Level 40 of Zone 4 - You do unfortunately have to finish the entirety of Zone 4 to get here if you restart from that point.
The ‘God of Pestilence’ is Boss 4. The balls you shoot aren’t strong enough to hurt this boss, so you’d normally use the bombs to defeat him. However, for this achievement, you need to shoot those balls at him 20 times in 20 seconds. Use the bombs to create gaps to get to him, but be careful that you don’t defeat him.
Remember, you have infinite lives when fighting bosses! If you think you may accidentally defeat him, just let the round end - it won’t impact your score or lives. But if you do end up defeating him, you will need to restart that entire zone to get to the boss again, so be careful.
This achievement is really difficult so be patient and try to avoid his attacks - good luck!