You can increase your wits by doing stealth kills (hold B for a short while until the character enters stealth mode - ie: crouching - and then approach a zombie from behind, holding RT + X to do a stealth takedown) or by searching containers.
Once you have all the stars in Wits it'll allow you to specialise - press down on the D-pad, find the skills menu using LB, and then select the skill and choose a specialisation option.
Once you've specialised the achievement should pop.