×Your first goal is to clear up to the 11-column. You can push the tar back south of the 3-long wall to give you a few spawn cycles without worrying about it.

If you're able to make enough progress to push all the tar to the 13th column or further, you can leave a few babies south of the 3-long wall.

It's relatively easy from there, but it requires lots of efficiency up front.

×You can trap babies at (20,11), (20,14), and (20,17). Then you can trap some at (10,20), blocking off the south portion of the room.

You should be able to make it from there.

×The point where the tide turned for me was when I cut the tar at (13,21), leaving a 2x2 blob to the left of it, so that I could eliminate that blob after it has grown once. I also left a bunch of babies under the 1x3 wall. After that, I've also stacked babies at (13,18).