This is a lot easier than it sounds. If you are playing ragequit rex, start in easy, which lets you take a hit before losing a life. Then if you do get hit, restart the level by hitting 'select' on you gamepad or the equivalent keyboard function. This will instantly restart you at the beginning of the level and you won't lose a life. It's still tough, but if you have already beaten the 3 star challenges on all levels, you should be familiar enough with the levels to be able to finishing them without getting hit.