Garen's Q, E, and autos all do %maxhp true damage. Lategame when his E's doing 10 ticks of damage + his Q that's at least 11% maxhp true damage not counting the at least 800 damage (I'm assuming this Garen at LEAST built Black Cleaver, full tank Garens make me sad) THEN his true damage ult and you have a dead Riven
If a Garen really freakin fed a Riven that means:
- That is actually a legitimately bad Garen...like really really freakin bad
- That Garen is maybe not so bad, but is very very seriously on tilt
- That is a very very good Riven who probably deserves the win
- That Riven was a smurf
- Some combination of the above 4