Try to combine your skills with those of your allies to do a lot of damage in a short time "ask the Jungle or the medium for help if possible".
If you don't have the ability to kill it, don't try to wear it down little by little; as it will recover quickly.
It is almost mandatory to cast the Ignite spell when Dr.Mundo uses his ultimate; to greatly decrease your healing.
If you want to beat Dr. Mundo in a simple way, you must combine two things; a champion that can withstand their confrontations and that generates explosive damage that can eliminate it, if to this we add some effect that cuts its life regeneration it will be able to tell it relatively easily