In PnP, he was a level 12 NPC boss with a couple of Warforged Juggernaut levels in there (can't remember exact class spread, but nothing granting evasion). I can understand how this would be insufficient for Turbine's intentions, but in what way could they justify giving him rogue/monk/ranger levels?
Evasion should be reserved for NPCs with obvious class levels in classes that grant evasion or of a race that is superhuman in its agility. Quickfoot wizards are slightly understandable, they probably have a few rogue levels. Wildmen are less so, though I guess by some strange interpretation they might be counted as rangers. However, there is no explanation for the LoB getting evasion beyond "we wanted him to be really, really tough." A nice thought, except that most anyone is going to use against him is DoT spells.
Please, please stop giving evasion to mobs for "balancing" purposes. It doesn't work very well a lot of the time, and detracts heavily from the feel of the quest - a good rule for the DM is that if you can't find an excuse without the word/prefix "meta" in it, then you should probably rethink the mechanic. When I get to the Lord of Blades, cast something, and see "evaded" pop up, I'm not going to think "yeah, that makes sense" I'm going to think "This is a big-ass robot that I just hit in the chest with a bolt of lightning. How the hell did he dodge that?"