Maester Seymour

Most counter payoffs in green ask you to bring a board and then multiply what already exists. This one seeds the counters itself. The combat trigger reads Seymour's own power and dumps that many +1/+1 counters onto another creature you control, which sounds unremarkable off a 1/3 base until you trace the feedback: the monstrosity ability's X counts every counter across all your creatures, so the pile you build with the combat trigger becomes the fuel for a single explosive monstrous swing. The design tension is deliberate. Seymour cannot grow his own power from the combat trigger (it targets another creature you control), so the engine only turns once you have a partner body to receive the counters; the counter source and the counter recipient have to be two different creatures. That split, plus the three-mana body and the five-mana monstrosity activation, is what keeps the exponential from arriving too early: you need a second creature, an open board, and time before the loop compounds. Monstrosity is the ceiling here rather than the engine. It is a one-time action per creature, so the payoff is a single decisive conversion of your accumulated counters into stats, not a repeatable pump. And because X is the total number of counters among your creatures regardless of how they are distributed, the shape of the board underneath the swing does not matter to the math: what matters is how many counters Seymour has manufactured before you pull the trigger.