You stated "Hit R and kills you" with absolutely 0 context, of course your statement looks as if you're claiming them being able to 100-0.
He must have enough damage to be able to whittle a single opponent down to be able to execute you person with his ult. And in order to do so, he must be able to stick to that one person since he has close to no mobility (Q is a speed boost that can be slowed or pushed off)
Simply a better Garen, but also faces the same problem in terms of sticking to an opponent. The only way he can "gap-close" an enemy is by catching one with his E. If he were to miss, he becomes highly vulnerable to kiting, which is why champions like Jayce or Quinn counters him hard. The only way he can "one-shot" you with his ult is for him to be able to hit you 5 times & then whittle you down to at least 25-30% HP. Believe me, it's actually harder to do so than it sounds.
Just CC the dude during his ult. There's a reason why he's a complete shit pick in pro-plays since 2014.