he is a tanky juggernaut. his W passive and active allows him to do this.
you need to find away to keep is passive off. poke him somehow to disable his passive so he cant out sustain you. he is going to be tanky, penetration items are good. healing reduction items/abilities are good vs him.
his trade combo is very simple. he wants to Q and then go in with E for the most of all trades. bait out his Q and make him waste it, then trade. if he uses E get out of it before it shreds too much of your armor.
if he is lvl 5 close to lvl 6 or has ult and you are below 50% hp you need to recall cuz he will all in you to get a kill.
his ult does magic damage unless you are the villain then it does true damage which scales on your missing HP
there is two items that help nullify his ult

nullorb in the sorcery tree is a nice pick up vs him as well
more so what champ are you playing into garen mostly?