Dr. Strange is so over powered on current situation that he can defeat everyone so fast with better obelisk(damage increase). He can freeze time, it means if this happends you can't do anything except to see him to reduce your character's hit point.

To get Dr. Strange fully ranked i.e. 6* you need to accomplish some quest which is called "Epic Quest".

There are total 45 quest. And some quest gives you good rewards like Crystals, Comic Card, Obelisk etc. Through this quests you will get him 6*. After completing some quests it will unlock two missions "Memory Mission" & "Dark Mission".