You don't even need the lightening to beat Demise. Just get close, shield bash, and do exactly like you did in Phase one, but with better timing. Keep slashing and you will evade his sword and hit him. If you were good at Ghirahim and hitting him when he was "blocking" with his sword, then you should be fine with this method.
The Lightening method is more fun. Try getting close so Demise shoots his strike at you, dodge it, and put your sword up immediatly after. He WILL charge you, but you should get the stirke charged in time to hit him. Good luck!