Steps how to reproduce:
1. Start Hexen as Fighter
2. Load MAP04 Guardian of Fire
3. Warp to coordinates X= 670, Y= -880
4. Kill the FireDemon via console (the ettins mustn't wake up!)
5. Face the chain switch
6. Give yourself grenade flechettes (give all/give artifacts)
7. Throw a flechette in such a way that it bounces off the floor first before hitting the wall
8. If after bouncing back it directly hits an ettin it may die at full health, even though they have 175 HP and flechettes only deal 128.
I recommend making a quicksave between steps 6 and 7, this can take multiple tries.
Note: I remember this already occuring in previous GZDoom versions. I'd have completely forgotten about it if it wasn't for Doomreal. The Autocannon deals 2400 damage total with the altfire, yet I managed to kill a maulotaur (3000HP) in one trigger pull.
Note 2: I enabled DamNums' Shotgun Spray option to double check and indeed it shows. Credits to Marisa Kirisame for the idea. Screenshot below.