Kicking Hulk must have been painful for Batman, not Hulk. Batman is definitely not strong enough to hurt the Hulk, Len Wein knows that, so I don't believe Batman's kick really hurt the Hulk at all, just broke his concentration of holding his breath since this is the stupid Hulk (without Banner's intelligence), and then the sleeping gas (which "can flatten a herd of elephants") put Hulk to sleep. I'm glad Len Wein didn't resort to giving Batman some hypnosis powers. Like David versus Goliath, I enjoy powerless Batman going up against powerful appoints and managing to take them down with no superpowers.