>>713514191They are about the same. Sans is not really that insanely difficult on an objective scale, he's just significantly more difficult that anything else in Undertale. ZeroRanger's normal stages are not that difficult either, it depends on how risky you're playing and whether you are using any of the defensive weapons that erase bullets. ZeroRanger's final boss and final challenge are both harder than Sans but that's mainly because you don't have any HP or healing and lose a life in one hit, so you have to play perfectly. An average player should have no problems beating it with some practice.
There's also a midgame boss that's a clear reference to Sans (
>>713510660) you could look up a video of if you want a point of reference.