Big Bertha

The X is doing double duty here, and that is the whole idea. Pay it up front and the counters ride in the moment she does, a green Hydra-style body that scales with whatever mana you had left; or pay a little now and grind the rest out one counter at a time through the tap ability. Vigilance stitches the two modes together: because tapping to grow her never costs an attack, she keeps swinging while the pile climbs, so the growth options are not alternatives but a single ramp curve you feed from both ends. That the counters go on a legendary body, not a spell or a token, matters for how green typically wants to use +1/+1 counters: this is a permanent that both hoards them and stands to double or move them under the right effects, a stationary engine that happens to also attack every turn. Left alone she is a slow accumulator; the payoff comes from the density of counter-multipliers a green counters shell already runs, where each activation is worth two, three, four counters instead of one. The design is unglamorous on its own and deliberately so: a mana sink with no ceiling, built to be the anchor of a deck already committed to making counters mean something.