Noticed this issue/bug (Edit5: Might not be a bug, might be intended mechanic - see Xerte's comments below) during my first run in trial 4. I immediately wished to test it more thoroughly.
My theory:
As long as you have your shield up for zebra/cardes' threshold attacks, your shield will disappear nearly immediately followed by a chunk of only 1-3k hp from your units.
 
Background: I re-ran it, even after killing cardes (force quit just as I accidentally killed off Phee and Luther) using team of Tridon leader, 2x Tiliths, Darvanshel, Falma, Belfura.
  Test: I used a somewhat scientific method of trying to narrow down possible causes for the ridiculous damage decrease:
At 50% HP I made sure that i had NO Tridon shield up, Darvanshel's BB up, and Falma's SBB up, everyone else on attack.
This meant there was a 60% mitigation on everyone, along with 70% boost to def. (I was trying to see whether perhaps it was mitigation def which somehow curbed the fixed damage).
The result was instant death casted onto Falma.
  Next test was:
On Zebra's 25% threshold:
I casted Darvanshel's BB, Falma's SBB, everyone else on attack, Tridon did SBB shield.
Result was, the 2 units that were targetted, Tridon was left with 7582/10136 HP and Darvanshel with 11110/11866 Hp
  This result, with the only variables changed of zebra's threshold attack (45k vs 50k damage), Tridon SBB shield down vs Tridon SBB shield up and target/s (falma vs Tridon Darvanshel), shows that there is a likely probability that the determining factor was the Shield, which in fact mitigated over 20k damage.
  I can do further tests if necessary but my aforementioned conclusion seems reasonable.
Feel free to ask for any specific tests if you want, I'm not in a rush to obtain/ level Cardes. I will be using him as a punching bag for team testing and other miscellaneous tasks.
  Thanks for reading.
  Edit: Cardes' Deadly End ST attack did over 7k damage on a mitigating 70% def Darvanshel (no tridon shielding). I will try to see if this is affected by Tridon shield or not. Testing continues...
Edit2: Please upvote for visibility~ I'd actually want this bug to be noticed and fixed xD itsss sooo strronkkk
 
Edit3: OH BOY check out Xerte's reply in this thread, Tridon is more OP than we ever thought. First hit of a Fixed Damage attack (typically shown by threshold attacks) is negated by the shield. Totally. Up to infinity damage. (the 2nd hit onward do normal damage). He is a g.damn premier anti-boss unit (who do fixed damage attacks like insta-ko stuff).
Edit4: So explaining further, according to Xerte, the hit that does enough damage to down the shield will be totally negated ( I believe ). So a fixed attack that does 1k damage, 1billion damage, 1k damage will have the 1k absorbed by shield (which has over 1k hp), the 2nd hit will break the shield but will be negated, and the 3rd hit will land on your units' hp.