You can find McDuff in The Lost Bastille just after the section where you encounter the pursuer and some undead/dogs in a small courtyard. To access his room simply trick the Enemy at the top of the nearby stairs into rolling the barrel down at the wall, thus destroying it and granting you access to the McDuff bonfire and the man himself.
To earn the achievement you will need to first give him a dull ember to access his services, then you will need to spend 14000 souls upgrading your weapons and armour, after which exhaust his dialogue and he will give you a titanite slab.
Achievement Unlocked!