This build is made for fun and not pressure tested.
Inspired by: https://www.bilibili.com/video/BV1Vg411P737?share_source=copy_web

Why mid lane?

First Strike on Garen? A terrible idea. Garen has difficulties even getting close to the opponent laner let alone getting the first hit. We have to accept the truth, you are going to be weak like a wet rag in lane without an actual functioning rune and you sure aren't surviving the hellhole known as top lane like this. So the logical solution is to migrate to mid lane where you are unlikely to be tower-dived by someone over and over again even if you are losing lane.

Why is this build atleast playable?

Garen has a high execute threshold and high base damage. All you need to do is to farm and survive lane until you get GALEFORCE, then the fun begins.
With galeforce you can reliabily proc First Strike and all in someone immediately to get that money. With this setup everytime you get a full combo off you are going to get about 150 dollars everytime which is ALOT of money. This allows you to snowball and with your runes your Galeforce's cooldowns will only be at about 45 seconds even during early game.