I seriously hope I found a use for finduin here xD using finduin father winter's helper and Kun-Lai runt. Yup so finduin start of with stun the reflective shield, and spam blessed hammer. Blizzard with father winter's helper and deep freeze and takedown with Kun-Lai runt. It took multiple tries to figure out the right combo. Plus enemy got lucky crits so horrible.
Didn't have any of the third pet choices so I used Kun-Lai Runt P/B with 2/2/2 instead
I followed the script above (though my pets was killed earlier than the script said) and when my runt came in I used frost shock -> deep freeze -> Takedown. I was probably lucky and you may need to be lucky but it is possible :)
My team: Deathscreech:3T0B:222813T:22231SF:222A14E:
Repeat Takedown if he doesn't kill you, repeat Frost Shock-Deep Freeze-Takedown with second Runt.
I got lucky as Deathscreech practically killed himself by criting on Murkalot's Reflective Shield. This strat will NOT work if he doesn't cast Greater Apocalypse 1st or at least within first 3 moves, or if he crits on Kun-Lai Runts when they are not shielded. Still only took me 3 tries to kill him again so RNG is not too bad for this strat.
After trying every strat here without Wild Magic, I went with the one below and got it on first attempt. The big RNG part of this fight is that the flyer is strong against humanoid, which means if he hits in the upwards of 1000+ you will not get 3 turns which is what you need.
Father Winter's Helper 2/1/2
Gift of Winter's Veil (you need to land this hit, restart if he kills you pet first)
Punish ( Grumpling dies here )
Swap to Terky
Stampede ( you need this 2nd hit to land as its double damage )
Stampede ( Terky dies here )
Gift of Winter's Vale ( should knock him down to almost dead )
All of the strats in here rely on 1) no crits, 2) apocalypse casting 1st or 2nd, and 3) infected claw always being on the low side of the damage range. Have patience, they all will work by the 2nd, 3rd, or 100th time :-)
Based on the pets that I have available and a lot of experimentation of my own, I modified this strategy with some hints from the comments and my own ideas. I ran Anubisath Idol (2-x-2), Squirky (x-2-2), and a P/P Bonkers (x-x-1). Note that this strategy is heavily, heavily RNG-based and will likely take many restarts; however, at least you should know within the first couple of turns if you need to restart (unless you get hit by a really unlucky crit later) so it goes pretty fast. Idol must get off the Rupture with a stun plus three hits with Demolish in order to bring Deathscreech low enough for this to work.
1: Rupture. If Rupture doesn't stun Deathscreech, restart. If Deathscreech crits, restart. 2: Demolish. If demolish misses, restart. Deathscreech is stunned. 3: Demolish. If demolish misses, restart. If Deathscreech crits or if Idol dies, restart. 4: Demolish. If demolish misses, restart. Idol dies. Deathscreech should now be just under 1000 hp. Bring in Squirky. 5: Bubble. 6-8: Stampede until Squirky dies. 9: Haymaker. Deathscreech dies.