I'm Sorry Everyone

I think the correct ban when facing this is Crimson Sigil Runner to deny the otherwise astounding draw engine and hope that you can deal with the Battlefiends.

NOTE 1: The evil turtle known as ShadowsOfSense mentioned a version where you run Altruis the Outcast instead of Aldrachi Warblades, and after the necessary testing I've concluded that the Altruis version is far better. It also changes the ban advice quite a bit as I'm no longer sure which would be the correct ban. If you're planning to win the board, it has to be Altruis. In the mirror I'm in favor of banning Twin Slice.

NOTE 2: I'm more and more confident that the correct ban is Twin Slice, as it denies Battlefiends from getting out of hands quickly and makes Altruis a lot more clunky.

And since at least some people seem to be playing this deck, maybe I should give some actual matchup advices as well:

Mages: I've seen two versions: Giants and Flamewaker. I usually ban Sorcerer's Apprentice anyway (surprising, ain't it).

Mechs (mostly Hunter): I'm getting more and more confident that I've cracked this matchup. Ban Metaltooth Leaper and mulligan hard for Battlefiends. Play fiends and Twin Slice 'em up right from the get-go. Kill the first Mech that drops, facetank over minion trade (Twin Slice is strong once again). When the Mech wave hits, ignore and go face and pray that you have enough damage next turn/over next two turns to finish the job (the not-Attack-buffed Mechs are forced to trade as they're not going to threaten for counter lethal any time soon).

If it's not Hunter, I usually ban Mechwarper to make the wave a bit slower.

Priests: Next game please. They usually run something super-annoying. I tend to ban Renew if they have it but I'm far less certain that it's the correct choice.

Druids: If it's Taunt-Mill, next game please. If regular Mill, you have to get lucky and assemble a massive Twin Slice/Altruis combo to win. It's going to be hard regardless.

Rogue: I've met couple of Counterfeit Coin/Flik Skyshiv Rogues and I find that hilarious. They usually have Doomsayer as well and I'm soft-forced to ban it as I rarely have a way of dealing with it on-curve. There are some Pogo-Hoppers as well, if they get many in opening hand, you're likely going to lose unless you feel you can ban it.

Can't remember any other prevalent deck type as of now but I can write more if I meet some.