One of the more interesting takes on the theme I've seen, and definitely a fun game to sink some time into. Controls could be a bit iffy sometimes with hit detection but relatively fine. The difficulty curve definitely ramps up to madness trying to stop the level 5s from ending the game which is fun. A few finer points:
- The game leads to a lot of spam. Since there's no downside to hitting EVERYTHING on screen as fast as possible by spamming back and forth it continues to get kinda boring since that's the best strategy. So mix it up, throwing in some sort of dangerous thing, some sort of (as suggested) way to remove cells that you can also destroy to cause you to think more about your actions.
- The map feels overly powerful since generally I was able to find any level 5s near instantly and destroy them, though it did get sorta crazy later on which made it a lot more useful and almost needed so I'm not quite sure what you could do there. Maybe don't show cells you haven't seen? Only show updates on seen cells etc. Just a suggestion.
- Varying up the cells could possibly lead to some more interesting gameplay, though I'm not quite sure what types you could do. That's up to, just my recommendation to add more :) (very vague I know).