I'll just give an update as to how I've done it the past few weeks at 80.
First, you do not have to kill Garr. Since the patrol's aggro radii decrease as a function of your level he can be killed fine on the ledge outside of the tunnel in Garr's room without aggroing the corehound pats. I still remain stationary and eat the infernos.
Second, as far as spec I still had to do it as ret with some points in holy. I tried as protection but was not able to keep up with the mana and healing demands. Since my ret gear is sub par, I use tank gear along with FR gear and just kill him with judgements, shield slams and crusader strikes.
The last time I went he was down in 18 minutes.
_________________
http://www.youtube.com/user/mystafide