The problem is Devastation requires off-hand, so how does the shield in the build even works?
Are there some specific shields that can be considered “off-hand” ? If yes, how do I know? There is no text specific that the shield can be used with Devastation.
Without the link to the actual thread you got the build from it’s difficult to say, but I suspect there may be a second set of weapon/off hand as well which would allow devastation to be cast when you swop over to them. No shields work with devastation.