[NooneAtAll3]

I swear I remember that when you get to the heart, you get to see something like "you need [huge number] of total damage, damage that has been applied [medium number gets increased by your measly tiny damage]" or maybe "health left [huge non-round number gets decreased by very little]" and then your character dies

I just assumed that it's some online thing where it counts total damage from everyone to finally slay that heart "together"

[StopDisinfo910]

You get your score shown as damage once you beat the act 3 boss(es) and then go on to act 4 to fight the final boss if you properly collected the three keys which unlock it while climbing.