Well, to kill him in a fight, you need to counter-build his tankiness.
So that means your physical damage dealers need to get items such as
(AD fighters and their ilk) and
or
(marksman) to counter their armor. Basic attackers such as marksmen and champs like
and
(Basic attack focused fighters) can get
to counter his high health (% dmg v high hp).
Your AP/magic damage dealers should be getting things like
and
(to counter high MR and high hp respectively).
Beyond that, Garen's lack of dashes means he's susceptible to being kited. That is, being slowed down/stopped by CC and kept from getting into range of your squishy damage dealers. To this end, using your CC abilities to keep him off of your squishies is vital to beating him in a fight. Moreover, there are some items that help with that, already mentioned
for the % dmg, but that also has an active slow.
break slows when he activates his Q, but that has a moderate cooldown, and he is completely vulnerable to being slowed anew as soon as he's used it. So other items, such as
and
also provide active slows, and there's
and
which provide basic attack and and spellblade passive born slows respectively.
Finally, in the event that he still managed to reach your important squishy damage dealers, active shields and heals from items can keep keep them (and the rest of your team) in fighting shape. Items such as
,
and
all provide a way to heal allies or protect them damage with a shield (the first two are AoE, and so are quite helpful against
's AoE physical damage from his E).