Moomyung

Moomyung is a mixed bag. At mythi, he isn't all that great. But with fantasy and some upgrades+ennobles, he's quite a killer. His best feature is that he targets the enemy backline. This is very useful in getting rid of enemy healers and in the All Stats+100% rift stages with 4 SK/Adv and 1 very strong Archer. 
But outside of that? Yeah, unless you have fantasy and high upgrades, there are better mages.