Slimefiend Boss

Anyone know how to beat him, I thought quest bosses were supposed to be at a fixed level compared to your own combat level. So I would assume leveling up combat would just increase the Slimefiend's level too? He just smacks me even if I dodge every slam and use healing pots on cooldown. I also have tried to walk on top of him to delay his attacks. He is lvl 143 I am level 102