Ok, so I use reckoning mode under the circumstance of many enemies and an orange. I always add an exp potion and I typically use a precision and force potion to maximize the amount of damage to ensure I complete it in a timely manner. I always use on a boss no matter what as a lot of times the moment you kill the boss you will gain the exp for the quest as well which gets the reckoning exp bonus and potion bonus. So the following is the best way to sum it up:
The "Thresh" monsters are typically surrounded by 3 or 4 "Boggarts". If the thresh is red and the boggarts are yellow then you can gain quite a bit here. Start the force, precision, magic amplification (if you want), and the exp bonus potion. Execute reckoning mode. Kill everything and use reckoning on the thresh. You can gain as much as 5k of exp if it works out just right at 100% bonus.
Sometimes, you are lucky and find four or even more "Jottun" around. I also will use the same strategy as above and gain over 8k pretty easily with at least 4 jottun if they are yellow. Same strategy for "Ettin" as well given that some even have the orange Ettin Preist which has popped me to 8k.
As far as a boss is concerned. The only one I can recall exactly is the story quest near the city of Canneroc. You will meet with your fateweaver pal (as you can see I am terrible with names in games). When you get to the boss, execute the above strategy and kill everything and then perform reckoning on the boss. I took home 29k of exp because of this. There were a few other moments in which I got about 30k with this strategy but it is kind of a blur as to which times this was. A lot of side quest bosses are an easy 10 to 15k with this strategy.