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
Emperor Crab: Surge, Healing Wave, Whirlpool
Stitched Pup: Diseased Bite, Howl, Plagued Blood
Lava Beetle: Burn, Cauterize, Scorched Earth
The elemental Au, isn't too terrible and I think any aquatic would be ok. The Emperor crab I used was a high health one (Breed H/H) so it could take a few hits, especially since Au uses Gold Rush which is one of those damage the dealer and the recipient abilities. Usually my crab will be up with enough time to put a whirlpool on Banks, the Mr Wiggles pig, who has a really annoying multi turn coin drop that is strong against beasts. I tried using my spider but it was smashed. I ended up using Stitched Pup, and even though the heal still went off with my howl lucky it was an undead and got an extra howl in. Then I let my lava beetle take over to finish off Lil' B. I turned it to Scorched Earth and burned it, and glad there was a heal for me to use too.
However, on a subsequent week, I could not get it with the beetle! So I ended up using a Dark Phoenix Hatchling and that worked (even though my Conflag missed).
Corehound Pup, Chrominius, Lil Bad Wolf are good substitutes for Stitched Pup. I don't think a beast is good because of the coin damage is strong against beasts. Lil Bad Wolf is a good one because it's a humanoid and has a dodge - might have to level him!
The team I used to beat Shademaster Kiryn
Crawling Claw: Shadow Slash, Ancient Blessing, Curse of Doom
Water Waveling: Ice Lance, Frost Nova, Tidal Wave
Robo-Chick: Batter, Overtune, Wind-Up
Man I found that Fluxfire feline mechanical hard! It does call lightning and call turret so I used my water Waveling to get rid of it, but it churns out the damage (Heh, after alL i use that all the time too!). I'm glad I got to use Crawling Claw, I saw Cymre had used it and so had Yuuna, so I gave it a try and it worked really well. Robo-Chick was surprisingly good against the wolf, I never really used it before.
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 (though not used): 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 avoids. I had the Oily slimeling ready to go but my moth had gotten lucky with hits from Moth dust so I didn't get to use it.