Genesect PFL 008 vs Cornerstone Mask Ogerpon

I was just playing on PTCGL, Mega Venusaur vs Crustle. Opponent had Crustle forward with Cornerstone on bench with Rocky Energy attached. Forest of Vitality in play.

I had Meganium on the bench, used Solar Transfer to move 8 energy to Genesect for a total of 320 damage to Cornerstone with Heroe's Cape. Instead of knocking it out, it was shielded. Why? Bug Cannon is a damage attack and not an affect. Am I missing something here?