Wouldn't necessarily have to be in-game. It could also be part of stat simulator, to see how much a certain build would be able to deal damage to, or tank a certain player.
For example, you could create 2 builds in the simulator and see what your average DPS would be, average hit rate, slash damage, dual shot damage, cripple damage etc. for physical classes, and seeing how much magic damage your skills would do for magic classes (for spiritual bolt in particular it could be great to know since level difference is quite important there). Just a few ideas, it might be simpler to implement than an adjustable mob in-game, since you can already setup one build entirely, and damage should be generally easy to calculate.