They're not getting butchered. They are butchering the attackers. CG III Mechanized infantry are the ultimate defenders, and this includes vs both AIR and GROUND. Don't forget to throw in their 25% fortification bonus. Their 25% for the city/fort that is often on a hill. Cultural/Trees defense... Etc. If we were playing vs human opponents then that's different. But vs the AI... I prefer to just let the AI slaughter itself, because it's that stupid.