Done in a few minutes: here some tips to get it quickly.
- When you first enter the Memorial, searching info about your daddy (2 missions early), clean immediately all the places from the mutants (don't forget the basements). Doing so, the Memorial is already a safe place for the doctors.
- When you have to bring Dr. Li to the Citadel, you'll have to face several ghouls too. Get the ghoul mask in Tenpenny's Tower mission (please, have a look at my solution if you don't know how to get it) and wear it. You can kill safely all the feral ghouls you'll find throught your way.
Another little suggestion. Completing this mission will reward you with 700 xp. Probably you'll get a new level. This is a great point to get the three karma achievements: in fact the Citadel's doors will open and the doctors will enter, anyway you'll end the mission a few second later. So you have a great occasion to save the game, fast travel to a city (like Megaton, Rivet City, Underground, etc.) and act to change your karma (my suggestion is to get a neutral karma, so it's very easy and quick to increase/decrease karma with just a little donation/murder).
Now fast travel back to the Citadel, you'll see dectors entering. Got che achievement, reload, change karma and so on 3 times untill you got them all.
so if you're on 6th/7th, 12th/13th or 18th/19th level (with the "Here and Now" Perk you can gain one additional experience level), this is a great occasion.