How the Hell, is Tywin supposed to beat Renly even without wasting troops fighting in the Riverlands? He has 35,000 before casualties and the Oxcross host which isn’t described as very good troops. The Crownlands might be pushed for some more but it won’t be a lot. So how is Tywin going to fend off 100,000 with only half that number.

If numbers were the only factor that mattered, wars wouldn’t be fought, you’d just compare army sizes and go home. 

But more seriously, I would guess that Tywin would use the defenses of King’s Landing and the fords over the Blackwater as defensive multipliers, while using his reavers to try to cut Renly off from his supply lines. Because the thing about a giant army is that it requires an astonishing amount of resources otherwise it starts to starve to death.