Seems either Nexon derped or they nerfed Heathcliff considerably, but not his damage.
We got the JP version of Heathcliff it seems. JP used to have 4 levels of difficulty scaled by the normal levels (Basic 0-100, Int 100-300, etc), but they removed it and gave everybody Basic 1/2 way through their event for unknown reasons (my guess is due to some bug).