Lego promised us Weekly Boss Battles and Leaderboards right after the big Merlok 2.0 update in March, but nothing happened. The Boss Battles and Leaderboards appeared briefly after the April update, but things were buggy. Some time this week, it appears that Lego finally worked out the bugs.
If you are signed in with your Lego ID, you should be able to view the Leaderboards and fight the Weekly Boss Battle as often as you want to. Access the Leaderboards by clicking on the 3-tier winnner's podium in the lower left corner of the map screen. The Weekly Boss Battle is currently on an island just to the left of Knigthonia (the city in the middle of the map). If you are far away, a purple arrow on the screen will show you where to go.
Competition is fierce for a top spot in the Leaderboards. The top 10 kills were all completed in less than 26 seconds. The current #1 spot belongs to somebody who killed this week's boss in 15.1 seconds.
I don't understand how anybody can kill a boss that quickly and it's very surprising that everybody in the Top 10 is using completely different Nexo Powers. My best guess is that these Top Knights are at level 99 and have maxed out their knights' gear. Are they hacking too?