Hi! I can come up with optimized and dynamic system for you, but developing for Unreal Engine might be costly, depends on full project scope. The more you diverse from typical FPS stuff, the more you need to care about optimizations and networking workarounds. Though battle mechanics of Blade & Soul still sounds within what UE4 offers out-of-the-box, considering that you do not plan to add some fancy stuff on top. If you have some complete project scope in your mind already, you may get back to me.
Regards,
Konstantin.