To put it simply, Garen is a very good dps if built correctly, and his spin benefits greatly from both critical strike and armor pen, with a focus on armor pen. This means that by getting ghostblade and last whisper, that you will deal much more damage when spinning on your enemies, and the total bonus attack damage will be high enough to make both Garen's Q and E do significant damage. Mercury Treads for the boots because the CC reduction works very well with the CC reduction built into Garen's spin, as well as the speed increase with his Q to get away from ganks or a teamfight gone bad. Guardian's angel goes very well with Garen's passive on Courage granting him roughly 90 armor and 60 magic resist from one item and his passive, which makes you much tougher than you look, especially after courage is activated. The frozen mallet is good to hit opponents as they flee, which will allow you to get the full benefit of your spin for damage, as well as adding some much needed hp.

In teamfights, be willing to initiate, instead of the primary tank, because Garen is often focused anyway, however, with this build, when and if the other team kills you, guardian angel will allow you to come back and clean house with a quick silence to an enemy carry, and then spin in the midst of the enemy team.