For an update on how to level to 212, see this article.
In KoM warfare, the level of your Hero determines for an enormous part how well you do in battle, giving bonuses to both attack and life of your army. Normally, a Hero can be leveled up to you Great Hall’s level x20, meaning lvl 200 max.
However, it is possible to go over this limit because the Hero stops leveling once he is over that limit. which means that if you overshoot the limit by much experience in one go, you can theoretically go to lvl 212. The largest amount of experience in one go is obtained with the “Train with Gandalf” item (50,000 exp).
To get a lvl 212 Hero, level the Hero up by any means, but stop just before reaching lvl 200 (to be exact, within 680 Exp of 200). Make really sure you dont overshoot, because once he is 200, there is no way of leveling any further. When you are just below level 200, apply the “Train with Gandalf”. This will boost the Hero to lvl 212. Voila!
Alternatively, 212 or even 215 Heroes can be won in Gollum’s Riddle, certain chests and in tournaments, but chances are slim…Cheers!