The main obstacle is owning the Black Knight. You will need to purchase the Knights' Barracks for 40,000 credits or 1000 tokens. Of course you must also own Haakon (900 tokens; 36,000 credits if you have Premium). Haakon is also included in the Champions Play Bundle and the Starter Pack Bundle. (All content is now free.)
The Black Knight is perhaps the easiest Crossroads boss. He is invulnerable except when he is hit with his own spinning blade. Stay far away and easily dodge his fireballs. When the blade is near you, lightly tap
to send the blade back. (Holding
too long causes you to dodge instead.) The blade should hit his shield and leave him vulnerable. Shooting him with
will take off 1/5th of his health. Repeat 4 more times and you're done.
Because of the ease of this boss, this fight is another good candidate for
Note (July 24, 2015): According to reports in the comments, the Nemesis bugs have now been fixed.
Note (July 3, 2015): According to
reports, all the Nemesis achievements may be glitched after Title Update 13 of June 30. The bug has been reported in the correct place for the developers to see it.