As you may know the dlc for fallout 3 introduced some challenging enemies. These include:
-Feral Ghoul Reavers
-Albino Radscorpions
-Super Mutant Overlords
-Enclave Hellfire Troopers

First off i will say that the Enclave Hellfire Troopers are weak compared to the other enemies. They only need 2-3 shots from the L repeater and only 1 if you use sneak-crtical attack to the full advantage. There only strength is there flamethrower which can take down your health if you are careless and outnumbered
Difficulty 4/10

Albino Radscorpians to me, are not really that challenging if you use the right tactics. When facing scorpians in the wasteland i just use the walking back and gunning approach where i shot them when walking back. It clearly is effective against all scorpians. However, this weakness is easily covered if they are in greater numbers 2-3 where they can swarm you easily and use the sting which can kill you just as quick. Also they take forever to kill as there health is so high which is annoying.
Difficulty 6/10

Super Mutant Overlords are beast compared to their previous super mutants. There tri beam laser combined with thier armour and high health can keep you at bay during a fight for a long period. Making ammo count, if you are low and constantly diminshing your stimpack supply. Trully they are a test, however combined with a companion you can make short work of them with coordinated fire.
Difficulty 8/10

Lastly Feral Ghoul Reavers. What can i say they trully f**** you up.
Difficulty 9.5/10