I will never rule out Messi magic, if things start and he has one of the better performances...sky is the limit. Quite sure Umtiti magic is not happening though 

For me the problem for Barca is not even scoring those 4 goals, it will be much harder to keep their net clean. Only real chance for me is if PSG decides to go super defensive and defend lead from the first game and then after they are 0:2 the panic starts to rise and it gets difficult to start playing. Don't expect them to be that stupid though.