I hate Dr. Mundo.

So I've been playing the game for a good 6 months now, up until recently where I've been playing it a lot more, like a lot a lot, and there's one particular character that I absolutely despise in ranked. He's in the title. Recently in one of my games, I played kayn jungle, and it was mundo top, I was doing great I was out jungling the enemy jungle, I went 12/7 (iron 3) and I was easily able to kill everyone else, but not mundo, this man i kid you not was 0/5 and for some god forsaken reason this man would just heal and heal and it felt like we were doing no damage to him at all, we lost after that and I demoted back to iron 4, and im now in pain, someone give me tips on how to beat this piece of shit. please.