×-Go down the stairs with the clone first, so you can kill the decoy.
-Get three roaches on the southern pressure plates. Use the decoy potion for the fourth.
-Get the briar to grow one tile past the doors in column 9, then close the doors. The orbs on the left are all unimportant for this, so don't worry about hitting them before the briar consumes them.
-Blow up the briar near the top bomb, then lead a roach across the briar growth you set up before to hit the pressure plate near it.
-I blew up the second bomb but I'm not sure why just from the demo. Most likely it was to keep the briar from growing too far on my side.

×Before releasing briar, I've waited many turns for roaches to spawn, so one of the roachies can get to north pressure plate. The rest of the level is just triggering four plates and waiting for briar to kill queen.