If you want a build to try with this merc, try my "Bro'ken" composition, currently blasting through PVE & undefeated in the PVP training grounds. Includes guide: https://outof.cards/hearthstone/mercenaries/parties/32-broken/
For Substitutes: Malf - Guff - Bru'kan are the main ones I'd say you need due to the efficiency of the example Turn 1. The whole comp is built around sped up Nature snipe damage though, so as soon as you stray from that it will get a lot weaker - I tried Thrall, and unless you've levelled up his Doomhammer he just won't cut it really, as having to actually attack into people and take retaliation damage isn't great. Cariel would be an easy sub for Cairne, as she will be a frustrating taunt to come into play from the bench - maybe even Uther since you can target taunt. You could then roll into a Human or Holy build and bring in Anduin as a substitute for Brightwing's heals, or Tirion to frustrate with Divine Shields, or other humans like Jaina, Lich, Varian. Likewise, you could even run Alexstrasza & Warmaster Voone with Brightwing on the bench to create a little threatening Dragon package. PS: For more visual merc news, follow me on Instagram.
Also, I forgot to stress how important spotting a big individual opponents merc turn coming up, and disrupting it with Bru'kan's Muddy Footing (next ability is 6 slower) which again, combos perfectly into Guff's brambles. It means you can usually kill a big threat before it has a chance to pop off, rather than going for the slower greed of the higher Bru'kan DPS abilities. As for BTX and really aggressive boards; they will usually never expect you to taunt Guff (who even plays Guff?) so he's a pretty great counter to it alongside Malfurions roots - and when Cairne comes in you'll usually have the option of two taunts to confuse them. PS: For more visual merc news, follow me on Instagram.
Murloc's might seem rough, but I think in the right composition they can cause absolute chaos; especially with Mutanus & Murkeye builds being one of the few lineups that can counter the popular BXT starting lineup. Checkout this 'Call in the Finishers' theorycraft which hides Murloc combos behind the safety of Uther & Cairne taunts, reborns and divine shields, while releasing powerful Morgl & Sylvanas damage from the bench https://www.instagram.com/p/CUunWqes9DW/
Great in Mullahoo's "Heating up, Twice" fire comp https://www.instagram.com/p/CUilt3QM14w/
This 'Heating up, Twice' build by Mullahoo https://www.instagram.com/p/CUilt3QM14w/