My input is just by experience battling in the arena. Of course you will see a difference on Level 11 and 12, but not as much as you expected. On Level 13 the best Champions become Super Fighters and on Level 14 and 15 become Beasts!
That's why i advised you to concentrate on your best Champion and try to level him up as much as you can.
It's better to have one Champion (for example your Warrior Prince) on Level 13 (trying to maximize him) to do the hard work in the arena and the others on Epic form, than you have 3 Champions on Level 11 or Level 12.
A lot of players, who don't have many crops, choose to Maximize Gilded Knight, because it's the "cheapest" option. Of course, he isn't the best fighter and hasn't the potential to become Beast, but on Level 15 even him makes the difference as written in the forum.
Until one month ago i was levelling to Epic all the new dragons that i won. Then i stopped levelling them and changed my strategy. I maximised my best 2 Champions and 2 Diamonds to make my life in Arena easier and now i can go back and start levelling my other dragons too