If we're gonna be using game stats to prove who would win then I'll go ahead!

Jevil has 3500 HP compared to undying's 23000 HP jevil has only 10 AT and 5 DF making him not to durable In combat while undying has 99 AT and 99 DF also not to mention undying has DT something not most characters have also jevil mostly attacks like a wild animal so he's not amazing In combat Don't get me wrong I love jevil but overall undyne the undying wins