So, you've finished the game, got a completed game save file and are looking to see what new content there is in the post game. If you head to your kingdom, you should see a bunch of new sidequests, but let's save those for a little bit later. The first order of business in Evermore is that a few more research opportunities unlocked, mainly in the Outfitters, Weapon Workshop and Higglery. Of course, you might not be able to complete them yet, since some require the few citizens you haven't recruited yet.
However, the reason why you're here is to visit the Spellworks and make sure you have one of the spells from there. Look at the research list at that facility and see if you got Retrial yet, which is at the bottom of the list. As the description states, this spell lets you face kingmaker foes all over again and it is through this that you'll gain your first method for leveling your characters. You just need to know where to go to redo the battles against the rogue kingmakers. Luckily, two of them can be fought in the same spots as the first time (Bastion and Brineskimmer), but the other two are located in special buildings.
For Oakenhart, head to the world map in the Ding Dong Dell region and look north of the Crypt of the Cat Kings to see a higher ledge on which you can land. There should be a Trip Door up here (Cradle of Creation), provided you didn't make a pit stop earlier in the game. Inside the nearby Cradle of Creation is a plaque you can interact with, which prompts you to cast Retrial and open a gate with this kingmaker. A portal opens up that allows you to begin a battle against the same Oakenhart, but he'll start out at level 50 this time.
The other kingmaker location you have to find is for Longfang, who will be located near Goldpaw. South and a little west from the city is an area known as the Backwaters, which is where you will find the Cradle of Fire. Enter and do the same deal as above to spawn a portal that allows you to refight Longfang.
Refighting the Kingmakers
These rematches with the kingmakers are the exact same as before, except there is one special mechanic to them. Each one starts out at level 50 and upon defeating them, you can retry the battle. The catch is that the kingmaker's level will increase to 55. Their levels will increase by five each time, until they max out at level 120. You will receive a piece of equipment for completing a fight, which is completely random. Don't forget that you can skip the intro/outro cutscenes for each fight, so this is definitely one of the better ways to level up your characters (and much better than refighting the final boss battle over and over) for a little bit.
Of the four kingmakers, Longfang is probably the safest one to fight, as his battle is fairly easier compared to the others, despite having to deal with small periods of invulnerability. With Brineskimmer, you need to get the higmakers to get the flying drone, as it's required during the battle. For Bastion, once he surrounds himself with electricity, he becomes one of the biggest threats and can easily kill you if you get too greedy. Lastly, Oakenhart is annoying, since you need to worry about the big poison attack that requires you to run to Mausinger and Oakenhart likes to jump around a bit.