This loop is a masterclass in turning a creature's own power against itself, powered entirely by Agatha's Soul Cauldron granting Deathbringer Thoctar's lethal activated ability to Dr. Beverly Crusher (assuming she has lifelink from some external source to trigger her own text). Once Dr. Beverly Crusher can remove her own +1/+1 counters to ping targets and gain life, Dr. Beverly Crusher's replacement effect doubles the ensuing lifegain trigger to continually restock her counters. Meanwhile, Deathbringer Thoctar's presence in exile is what lends the Cauldron its engine piece, turning every iteration into a self-sustaining cycle of infinite damage and growth.