There is a problem with most of these Chesnaught's; they won't survive a hit from something like a Mega Gengar, which is why I use this:
Chesnaught @Black Glasses
EVs: HP 4, Atk 252, SpDef 252
The terms behind the use of Payback are amazing, hence why adding it onto something that can survive a hit from a Mega Gengar is great, especially when you actually use Payback, it becomes a OHKO! Also, the reason you EV train him in SpDef is because his Def is high enough already, this just makes him bulkier and more deadly.