Even if we forget the fact the guy was crazy and thus doesn't think naturally, there remains still the fact he didn't know Touma's power unlike we. To him it just looked like Touma somehow punched through his vector change defense, and it's almost certain an idea that there could be a simple ESP cancelling esper out there never crossed his mind, so he couldn't concentrate on that. From his previous, very numerous, battles he had probably learned that when your strike doesn't bring the opponent down, increase the power. So, like Arch said, he decided to switch to stronger and stronger attacks. For this guy who never really had any need to think about defending himself, only thinking of different attacks was the natural way. I don't think there was anything strange in this fight in that sense.