You are absolutely right, think of it in terms as close to reality as possible. First off, lightning has enough energy to split atoms, there is a HUGE amount of power there. Don't get me wrong, Mercer would totally be a challenge, as would Mcgrath, but in the end they would BOTH DIE FROM EACHOTHER. In order for Mercer to be able to chape-shift, his molecules would have to be unstable, able to reform to an extent, all Mcgrath would need to do is use the Lightning Storm power, once it hits it would be all over... for both of them. With Mercers molecules being as unstable as theyt are, the power of the lightning would cause them to all separate and split, causing a massive nuclear blast. In the end it would be a draw, but man what a fight it would be.