Board Thread:Questions and Answers/@comment-30088034-20170219194117/@comment-13682699-20170225193736

Flowey is a Soulless flower that was grown from the magical essence of a monster and injected with determination from a human. His magic is probably fairly potent with determination fueling it. He also possesses a physical body, even if it is a flower, which is tougher than the magic that monsters are made of (mettaton/dummies are a good example.) Not to mention that with his ability to save/load/reset as the being with the highest determination, it would make him virtually unstoppable against any monster.

However, his ability to affect humans is greatly diminished as humans in general are much larger, made of tougher stuff, and possess far more determination. His only advantage is his ability to cast magic, in such case the only person who he could potentially harm would be a child(such as Frisk) or a severely weakened/sick person and even then their determination would allow them the ability to save/load/reset.