Recommend doing this against a 2nd controller. Can simply login into another account or hit B at the sign in screen and chose the option to continue without saving.
It took me a bit to realize what I was doing wrong. Batman needs enough health that the initial hit from the Assassin Strike does not kill him. Get his health down to around the B in his name.
You need 1 bar of the Burn Meter. Hit Back, Forward
then
slightly after .