Sounds a bit complicated to me. And is it really worth it to discard an enemy after he attacked you? Usually, with Rivendell Blade you're able to knock almost every enemy out of the park with a single attack anyway.
I also don't see how this combo is supposed to work with Mumak, unless you have two Rivendell Blades in play. How likely is that?
Sometimes it's worth it if you don't want to deal with them another round or need to kill all enemies to win but yes it's definitely best used to simply dispatch enemies who already have 0 defense.
It could be quite likely if you want it to be and use cards to fetch attachments such as Bofur, Master of the Forge or just use classic heavy card draw. Or just start with Bard and you only need one Rivendell Blade. And of course, the Mumak being rare, by the time he does come out, if he even does you could by then have two blades in play but yes you are then building a deck specifically to get this combo rolling more often.