But wait, the suscribtion of the attack is:
"Inflicts severe damage but MAKES THE USER FAINT"
So I dont think that a substitude will change anything. A substitude has to do with HP, but the attack suscribtion sais the user will faint and not the user will lose its whole HP...get it? It's kinda logical, so I think the user will faint, also if he has the sub up.