How can I avoid the Giant Glowing Radscorpion's special attack?

From what I was able to discern, you're not missing anything; these enemies are just way over levelled.

Am I missing some critical strategy to taking these s***heels down? They've literally one-shotted a level 50 god-tier dweller at full health on 3 separate occasions. I'm talking "maxed stats at level 1, then trained to 50 with E7 wasteland gear" dwellers. Statistically perfect dwellers, and that pounce attack takes them from full health to zero in one shot.

However, the tactic (for the time being; until/if they get nerfed) is to use pets.

I switched my tanks from +damage resistance pets to +health pets, and it seems to have made a huge difference...radiation damage isn't affected by damage resistance, but it does do a flat amount of health damage, so health pets are WAY more useful than resist pets in questing. They inherently improve resistance to both types of damage.

I now use my resist pets exclusively on the dwellers I send out to scavenge by themselves, and use the health pets on my melee tanks for overseer quest groups. It's made it way easier to manage the radscorpions... or maybe I just haven't met the badass one again quite yet. Either way, it makes sense from a strategic standpoint.

Source

Short answer you can't. But it isn't necessary that your dwellers will die. If you have dwellers with max health, they should be able to survive if they have full or nearly full health.

Glowing Radscorpion appears in 3 instances:

1. (Storyline quests) These are usually weak and can be easily taken care of.
2. (Daily/Weekly quest) These may appear as part of a group (1 Glowing + 3-4 normal Radscorpion). These are strong but if you patch a dweller just before the special attack, it should be able to survive with 10-20% health.
3. (Lv50 Daily/Weekly quests with ~20+ DMG weapon) Sometimes a solo Glowing Radscorpion will appear. By now you may have noticed in this quest enemy are mostly appearing solo and with health rivaling those Alpha Deathclaw. This solo Glowing Radscorpion will 100% one shot your dweller irrespective of whatever outfit/pet/weapon/skill he/she is equipped, if you don't kill it before it uses its special attack. To make matters worse, it'll have health 3-4 times that of a normal Glowing Radscorpion. So unless you hit with 3 consecutive 5X critical hit, prepare to lose one dweller or the entire party.