I hate to say it but... Also, once you get enough boosts(you can get a lot from task completion and upgrading your palace) you can keep boosting oracle knowledge upgrades (for example, I got from halfway through level 14 to halfway through level 15 by upgrading the Cavalry Offense upgrade to level 3.

I hate to say it but... I finished the level 10 quest in a day because I grinded out tasks and building quests, but the level 15 is a lot harder. If you keep up with the grind, though, it works. I just got to 15 today after 4 days following my completion of the first quest.