Are you serious? Garen has one offensive item in that picture, and the damage he dealt that scales with offensive items was eclipsed by Nautilus.
Seriously, Garen's scaled damage from items there is around 300 damage. That's not OP, and that invalidates your point about him having one offensive item.
Now, you got hit by an execute. Executes scale with your own missing health. I believe Garen is the only one with true damage on his, albeit situational true damage, however even if you were not the villain you would have died to his magic damage. You can't argue against this, you had 50 MR.