So just for fun, as I've now gotten all the level/alignment achievements, I decided to try and kill Fawkes, who's waiting in Underworld. I knew he'd be tough -- when VATS shows a mini-nuke barely scratching him, that's reason to pause and think about what you're doing.
According to the Fallout Wiki, though, Fawkes only has about 500 hit points -- less than I do at this stage of the game. Yet -- and I admit this may be due to my own bad aim, as I usually try to do combat in VATS -- it seemed to take me 47 mini-nukes and 4,000 Electron Charge Packs for me to kill him. (And I was using stealth boys and doing as much as I could as "sneak attacks" for criticals -- 100 sneak, 75 big guns.)
So, aside from the obvious early way to kill him (with the terminal outside his cell in Vault 87), what's the easiest way to kill Fawkes?