The final triad was this week! And what I didn't realise was that you don't have to defeat them all at once to get Master of the Masters, as I got the achievement whilst trying out my various teams to figure out what worked best.
The team I used to beat Blingtron
Spawn of G'nathus: Swallow You Whole, Lightning Shield, Thunderbolt
Cinder Kitten: Rend, Immolate, Scorched Earth
Corehound Pup: Thrash, Howl, Burn
Au is an elemental who has a charge attack so I start with Lightning Shield, then Thunderbolt and then Swallow you Whole until Au dies. Sometimes Spawn dies as well but usually Au's health has gone down enough that Cinder Kitten can kill it on the next turn wtth a Scorched Earth.
Banks is a critter who will usually start with Make It Rain (coin drop multi turn attack) and then Uncanny Luck and heals itself with Buried Treasure. With Scorched Earth up, I just Rend Banks till it dies, and if I can, refresh Scorched Earth before Cinder Kitten dies.
Lil' B is annoying because of the Extra Plating reducing the damage taken and also has a heal with the Blingtron Gift Package (when it misses). But with the Scorched Earth doing elemental damage and Core hound Pup using Howl and Burn, it should finish it off easily!
Spawn of G'nathus - Electrified Razortooth or a high health Emperor Crab.
Cinder Kitten - Molten Hatchling
Corehound Pup - Dark Phoenix Hatchling, Phoenix Hatchling, Pandaren Fire Spirit
Yuuna's team: Electrified Razortooth, Cinder Kitten, Corehound Pup
The team I used to beat Shademaster Kiryn
Crawling Claw: Agony, Death Grip, Curse of Doom
Water Waveling: Ice Lance, Frost Nova, Tidal Wave
Robo-Chick: Batter, Overtune, Wind-Up
Nairn is a humanoid that has an attack named after itself that is strong against undead. He always starts with Call of Winter (Blizzard). So I start with a Curse of Doom then Agony then Death Grip which will bring Summer (Beast) into play. I swap into Robo Chick and Wind-Up and then Overtune, and use Wind-Up again because Summer will use dodge after that. Then I use Batter until either Robo-Chickis dead or Summer. Then I swap in Water Waveling. The Curse of Doom and Agony would have been ticking on Nairn and killing him in the back row so he should die after another attack (which I usually use Ice lance since it's still a Blizzard and will do double damage). Stormoen will start with Lightning Storm which will remove the Blizzard so I use Ice Lance and then Stormoen's next attack will be Build Turret. I use Tidal Wave to get rid of the Turrett, then Frost Nova and Ice Lance to get rid of it. Crawling Claw is still alive, so if Water Waveling dies, then Agony and Death Grip should get rid of Stormoen.
Crawling Claw - Restless Shadeling
Water Waveling - Terrible Turnip
Robo-Chick - Personal World Destroyer
Yuuna's team: Crawling Claw, Fiendish Imp, Lava Crab
The team I used to beat Wise Mari
Amber Moth: Slicing Wind, Cocoon Strike, Moth Dust
Clockwork Gnome: Metal Fist, Blitz, Build Turret
Oily Slimeling: Absorb, Corrosion, Acidic Goo
Wise Mari was not too hard, my Amber moth was a showstopper as usual and took down two by itself with lucky avoids and big hits. I used my clockwork gnome against Spirus, the magic pet, which seemed to work well with turret and punch to get through its one turn avoids. I had the Oily slimeling ready to go and I start with Acidic Goo, Absorb then Corrosion and Absorb till one of the other debuffs drops off and then refresh it.
Yuuna's team: Chi-chi, Scourged whelpling, Oily Slimeling