Riot, you wanna explain why Mundo's self heal is completely broken?

Did an experiment last game where the enemy mundo let me attack him for a good couple of minutes, without any of his team, or mine, interfering. I was xin zhao, had 191 AD and 35 bonus magic dmg per hit on my jungle item, and a base 1.05 or something like that atk speed. I didnt use any abilities for 30 seconds, only auto attacking, and his health didnt even reach half after those 30 seconds. Then, i used my abilities, except for my ultimate, and his HP still dropped very slowly. That should tell you something right there. If i can wail on a champion for 30 seconds straight with a fair amount of DMG per hit, and the enemys HP bar not really change much, obviously there is something wrong. In my opinion, mundos regenerative abilities are completely broken. Hell, the whole time i was attacking him, he had his fire balls spinnin around, and those steal his own HP. So please RIOT, small nerfs to the champions you know are screwed.