First of all: sorry for sharing and trying to ruin other people's Brawl experience.

This list has been working pretty well for me, and since no one else has shared a Steamcleaner Plague DK, I guess I'll have to be that guy.

Card choices:

1. If you don't want to include Steamcleaner or Helya, play something else.
2. I really like 2x Down with the Ship, useful for early board control or fast unblockable damage + plague source later.
3. Staff of the Primus is not a must, but more plagues is more plagues.
4. I think the flex spots are Doomsayer and Distressed Kvaldir, so you can start teching against what you're facing by replacing those. Especially the latter has been quite useless for me, now that I think about it.

Mulligan things:

1. You usually want to have Steamcleaner in your hand. I've sometimes thrown it away if I think my opponent has one too and will be more prone to playing it. In that case I heavily prioritize Helya.
2. Target Doomsayer is you're fearing a board-battling aggro stuff. Seems to get rarer for me these days.
3. Try to keep either Doomsayer and/or at least one Down with the Ship against Mages - they seem to be going towards Mana Wyrm and other annoying things you don't want them to have.

Random matchup things:

• As mentioned, I toss Steamcleaner away against Warlocks. You want to let them play Steamcleaner first, and then fill their deck with plagues. Hope for Frost Plagues to ruin their follow-up combos - you can't win from hand in one turn. Forcing them to play Steamcleaner on a Doomsayer board is usually a smart move.
• In the mirror, I tend to let my opponent play Steamcleaner and then go full Plague on the response turn. Hopefully the portals have given something nice to threaten extra damage from board as well.
• I've faced very little Druid but the one I did I lost hard (Mill). Don't play Steamcleaner against them, just go full Plague. Save some Plagues in case they steamclean.
• Steamclean asap against Quest Mages. Remember that their extra turn from the reward does include drawing a card (= ton of plagues) so don't give up too soon.
• Some Mages have started teching Deck of Lunacy against Plagues and Curse of Agony - you're going to have a bad time when that happens. Try to bait that out with only a few plagues if possible.
• Aggro Hunter has been surprisingly tricky as this deck really doesn't challenge for board - hope for some good rolls and remove their stuff asap.
• Haven't seen any other classes in a while/at all, so I have no idea what they're trying to do.