Delixe: The Radscorpions near the Mojave Outpost? Jesus I took them out at level 4 with a half-knackered Varmit Rifle. God help them when they see a Deathclaw.
StingingVelvet: Level 16 with good weapons and decent armor and I still can't kill even one Deathclaw, let alone several in a row.
The big daddy mosquitoes kill me quickly as well, but I can usually pick them off from a distance before they get near me.
Go to the gun runners and pick up an anti-materiel sniper rifle, its a bucketload of caps (and the ammo is heavy as hell) but well worth it when you can one-shot deathclaws. Gives you the godlike sensation you got in fallout 3 when you had the alien blaster and the entire capital wasteland was your bitch. I've never used the armour piercing ammo for it but then if the normal can kill regular deathclaws in one shot, its hardly necessary. Sneak attack criticals are ideal with that gun, makes clearing the mine vastly easier (I did it solo so my companions wouldn't die)
Giant radscorpions aren't excactly simple to kill, its very dependant on your approach and your weapons. Get a varmit rifle and some AP ammo, hit their stinger from a sneak attack and they go down fast. Run in with laser gun blazing shooting their body and you'll get your arse handed to you because it very probably won't penetrate their shell. Much the same holds true for most of the tougher animals, Cazadores are another example of somthing you need to handle carefully because they can slaughter my level 28 character with ease if I don't attack them the right way
It sounds like someone isn't paying attention to how the game works and is trying to play it like fallout 3