If you are just starting out with Prince, you are going to take at least 5-6 days grinding for around 40 points in Dexterity, Magic and Strength and a few point in support and Seduction.
With Knowing and Partner, it is even faster because with Knowing, you can have 40 Skill points on day 0 without needing to do anything. And that mean you can start fusing out Gigachad on that same day. You can quicken the leveling up process with Partner, recruiting Aila and having her train the Seedling in random portal to level 4. 3 level 4 seedlings can farm training room battle and when they get to level 7 they can farm king. I typically get those gigachad seedling to max level one 1-2 days with Knowing and partners because of how fast they level up (Level up 4 times faster than any unit in game)
On challenge 10, instead of 3 seedling, you may need 4 seedling