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.
used rotten litte helper in first slot rest of slots just like top one, but had to have everything perfect, took me about 15 trys or more until it worked, tryed few other suggestions and still only worked on first suggetion, and only when my pets were not critted, and my pets did crit. and he has to cast apocalypse in attack first or second.
I combined two of the posts already here, made a modification or two, and found this one worked for me.
Rotten Little Helper 2 2 1 (mine is P/P)
Squirky x 2 2 (mine is B/B)
Bonkers 1 2 2 (mine is P/S)
One thing I learned through lots of trial and error is that if Deathscreech does anything other than apocalypse on his first turn, you might as well restart. Otherwise, RLH dies before he can get off the ice tomb. If DS does throw apocalypse first and all the stars are in alignment, then you'll get off Greech's Gift, Ice Tomb, and an Ice Lance. RLH will die, but Ice Tomb falls, stuns DS, then Sqirky can come in, bubble, and hit stampede. He'll die but DS should be under 1k in HP. Bring in Bonkers with tornado punch, dodge, and jab. It still took a bunch of restarts to get it to work, but it did work whenever apocalypse was the first strike by DS. Your mileage may vary if your pets are different breeds, which is why I added what mine are.
Worked the first time for me with a P/P Bonkers, and I didn't get an Ice Lance off for RLH. Greench's Gift then Ice Tomb, RLH dies. Bubble, Stampede, Stampede, Squirky dies. Tornado Punch, Dodge, Jab finished him. Thanks!
This is the only strat that worked for me... RNG was kind enough to let me get an ice lance off of RLH stunning the bat and allowing for a 3-turn stampede off of Squirky. When I brought it Bonkers, I did Tornado Punch and Jab and that was it. Thanks!