Just faced a Reaver (Fallout 3) for the 1st time. I was Level 15 & was just roamin' the sewers tryin' to find new locations. The Ghouls I was encountering got sliced to bits with my Vampire's Edge. My melee ability was only around 40, but still these Ghouls were EZ. Then this Reaver chucked some bio-goo on me & I figured I'd just hack him up. It didn't work. He trashed me. I started over & decided to get serious. I laid 6 mines (65 Explosives skill) & lured him to chase me. He endured all 6 mines & I still had to bust him up with the Terrible Shotgun a few times. Tough Character.
Those Super Giant Super Mutants are Killer too.
You should play the originals if you want an even harder challenge, one supermutant can ruin your day unless you have power armor and energy weapons.