Since your damage is useless late game try to get ahead maybe trade, fight, kill'em, ask for a gank, or stop them from farming. Remember warding becuase you can't kill their top and jungler both and living, unless you're fed as ****. Just Q one them and run if getting ganked, if running is not an option try dueling the lower health one if possible just get them low, ult, and run like you've never ran before. You can mostly out trade anyone as Garen if even or even if they have a kill on you just don't do anything dumb to get them more fed. Simple trade as Garen is W go in with Q silence them and spin (E) once spin is over just go back to farming unless they are low and you can get a kill. DON'T FIGHT IF SPIN IS DOWN that's all your damage and you will loose the trade if not up.