Board Thread:General Discussion v2.0/@comment-26347196-20151109173852/@comment-28046312-20160713000952

Sans, because you need decent platforming skills and fast reaction to win this battle. It's also pretty long and quite difficult.

Photoshop Flowey looks like hell, but it's easy if you know his attacks. Even if the battle is pure RNG, you have a lot of healing supplies which will help you.

It's literally impossible to lose this fight, because after death, your SOUL will regenerate and you'll be taken to the nearest checkpoint. Also it's not that hard except a few of his attacks.

Even if she's not a "final boss", Undyne The Undying is probably the second "most difficult" boss IMO. Harder than the non-genocide battle and sometimes you need to be pretty good at blocking these arrows, because they move so fast.