Board Thread:Questions and Answers/@comment-27327020-20160404174206/@comment-26887879-20160407142614

Well, isn't it implied that Monster's strenght/difficulty depends on their WILL to fight (you can find that in the Library of Snowdin). So no matter how the Monster is naturally strong, if they refuse to fight, their Attack and Defense weaken and are easier to kill. It is said that the Boss Monsters are the strongest, but neither Toriel nor Asgore really want to hurt you...

Asriel(combination of 6 Human Souls and Souls of every Monster in underground), followed by Omega Flowey are probably technically the strongest, but Flowey just wants to play around with you rather than end you for good, plus the human Souls he absorbed turn against him. You get to fight Asriel only in True Pacifist where you get every Monster's support and incredibly strong Determination since you refuse to die, but don't want to hurt anybody.

However, if the Monster's will to fight is strong, their fight gets extremely difficult like the case of Undyne and Sans in Genocide run. In this run, you're the main enemy, they know you'll destroy the world and knowing that if you continue trying, you'll eventually win, the reason their fights are difficult is they WANT you to give up.