Garen mid, was considered as a trollpick, why he is a decent midlaner now?
Changes in last patch, made him really incredible, u have good sustain, great damage and, most important, u have good matchup vs most of midlaners in this meta.

Garen is countering every, yup, every single one assasin, why?
Assasins are mobile champs, that deal most of their damage by their abilities.
What if we will take those abilities? After using q on them in late game, they are dead.
But, how i will use q, if their are so mobile? Well, building yoummu, helps a lot, and u get huge ms boost from your q, that last forever. After using q, they cant do anything, just use e, and finish with r, often u wont need to use it.

Control mages, but not everyone, Malzahar, Annivia are preety hard matchups, but Orianna, is very easy matchup.

You want to play marks, and quintessence lethality, with full build, u will have oveer 80 lethality- so u will ignore ~80 armor. Your e grants you % armor pen+ black cleaver.
If someone have 1 or less armor item, you will deal true damage.

You can play ghost or ignite, you can also pick exhaust vs Zed. For masteries you always want to take precision, if you want to have extra mobility, you can choose stormraiders.