Do the quest "You gotta shoot em in the head" (talk to Mr Crowly in the underworld).

Do the quest and get the achievement, then go off on another quest or do something else for about 3 Fallout days.

Make sure you have some stimpaks and a combat shotgun or a strong gun, go back to Mr Crowly and talk to him, he should have different clothes on.

Kill Mr Crowly, take the T51B armor and if the ghouls turn hostile, run out of there, or kill them all.

When you get back out,heal yourself and wear the armor.

Congrats, you have now the got some of the best and one of the rarest armors in the game!