i don't know is mercer could actually do anything to War. The regeneration is wats the killer. If there is nothing around for Mercer to absorb then war in a stomp.
And if we are talking about full power War, then all he would do is wait for mercer to reform and destroy him again. no getting tired out.
War in theory could go and destroy all life forms around the battle area then mercer couldn't reform. As for mercers speed, War throws glaive and holds him in place, then charges forward riding Ruin and that would be then end. War needs to get close. Also it doesn't matter what building mercer climbs, cause War can throw cars up to 30+ stories relatively fast.
I don't know a whole lot about Mercers attacks though, but from what ive seen, they wouldn't do much damage
A interesting match imo, two beings that both have a high level of durability, have shown considerable strength and are basically good at what they do.
I personally doesn't think that War will stomp Mercer. I see them coming off somewhere around 6/10 for War.
I have a question however, do we allow War to use his extra life points? Mercers ability to regenerate are a constant factor, but War's "regeneration" only kicks in when he is "dead" technically that would equal a win since it would then count as a KO.