Board Thread:General Discussion v2.0/@comment-26347196-20151109173852/@comment-27324545-20160106140234

Sans is definitely the hardest boss in general; you cannot lose against Asriel, and as long as you make it past the first SOUL, you can start from where you left off against Flowey.

No-hit difficulty, however, would go to both Asriel and Photoshop Flowey. For Asriel, I cannot avoid damage against Hyper Goner and the wispy attack on the second part. For Flowey, I cannot avoid damage against the flamethrowers, the finger bullets, and the second variant of the eye star attack.