infinite mass punch would generate infinite amounts of kinetic energy. and since lady death wants strong guy down by making him have a weak heart thanos granted strong guy immortality and unable to die due to health issues. thanos did that insite of strong guy, not becuase he likes him ie: thanos is inlove with lady death and wants to be the only man by her side.
so with a new undameagable heart. infinite mass punch vs strong guy.