×1) You need to keep the briar fully connected until you kill all the roots, so that the pits slow it down. (Diagonals count as connected.) The moment you break the chain, the northern part will instantly grow and close up again, probably killing you in the process.
2) Because diagonals count as connected, you can break through the briar and use the broken walls to attack the roots from behind. You can get through the briar after the first couple of growth cycles. (A direct assault on the roots also works, but takes longer.)