Short: Just keep your average dweller lever low and even Deathclaws do no harm.
Longer: Because of the HP\Endurance link I created a new Vault and didn't level any dweller (or tried to, as the level up arrows overwrite most other options). Now I have 66 badly equipped dwellers (because I can't send them out without leveling automaticly). Events last quite long because I upgraded all rooms once and killing stuff with fists takes time. But I haven't lost a single dweller. Even at level 1 they take like 5% damage from 3 Deahclaws. They even killed Deathclaws with their bare hands.
Same with everything else. Raisers still come in groups of 3 and carry swords. They never make it past the 2 entry guards. Fires do no damage at all. Same with Radroaches and Molerats. Rooms that don't level your dwellers (like storage and radio) are suddenly good to have.
Now i'm thinking to downgrade my main Vault as well. Letting 5 guys level to 50 and let the rest stay at level 1.
The worst thing happening in this kind of Vault is the arrival of a legendary or rare dweller.
Has somebody else tested this already?