How do I complete this by my self? And how do I make sure we don't lose?
you've done well!!
The plague Hearts arent meant to be done solo, but once you level up a bit more you can upgrade Sear to diminish healing and then you can solo them
Until then, the best thing you can do is keep them surrounded with 4 purifiers until another shows up to help destroy them
On this particular map, you have already secured the win by activating the grid. Which prevents us from falling below 5% clear spaces
Great job!!