The implicit beauty of this game is in finding ways to work in conjunction with strangers you can barely communicate with. Having said that, there's a Nash for a single player to reliably manage things nearly by themselves.
Pick a wall, left or right. Purifier it, top to bottom, and start walking to the far side. Slightly more complex, pick the least contaminated corner and purge it so as to build a diagonal wall of purifiers. Additional users divide the time to completion, so it scales, but one player interested enough to stay in the grind can eventually handle everything outside of the hearts themselves.
Where we're at now, the contamination is essentially everywhere, and a phalanx of purifiers is the only reasonable and intuitive tactic. We probably need more emergent behavior from the enemy to overcome this One True Way playstyle.
(I'm having fun, but I can see it getting old since I always feel at this point like I know exactly what to do.)