My proposed small buff for jungle Garen
's W passively grants him 0.25 bonus armor and mr for every unit killed, up to a maximum of 30 bonus resistances.
This means that Garen needs to kill 120 units to fully stack his passive, which takes quite some time if he's playing as the jungler (unless he decides to heavily tax his laners).
My proposition is this: why don't you buff his passive so that it grants him 0.5 bonus armor and mr (instead of 0.25) whenever he kills a large or epic monster?