I play support mostly, so I really only know how to deal with Garen during teamfights. Basically, when you see him running at you when he uses his Q, you can slow him all you want because now his slow remove is gone. Then when he does silence whoever, he's right in the middle of your team, and either that's the enemy team's initiate or Garen is attempting to poke. If it's the first, he's going to use his W and E shortly after, so you actually don't want to attack him. This is the time you focus their squishies. Soon as he stops spinning to win, he's either going to be easy to kill if he built damage, or he's not going to do any damage if he built tank. Yeah, his dunk is nasty, but someone on your team should have their CC back up around this time (After killing their heavy damage), and you can stun/cc him and kill him. Basically it changes around your prioritization list. It goes like APC-ADC (You may swap these depending on which is cranking out the most damage), Garen, Support, and then finally Tank.