User blog comment:General MGD 109/Who's most powerful?/@comment-66.186.252.20-20121028090702/@comment-75.26.152.251-20130723190005

I don't know if the 5th season came out by the time you wrote this, but the reason Merlin couldn't heal Arthur is because Arthur was stabbed by a blade forged in a dragon's breath, which cannot be healed by a sorcerer. Only in a certain lake can the wound be healed. It's not about power in that case, it's more about technicality, unfortunately.