I use an Android tablet, so my battle results may be different than yours if you use IOS...
As to Zodiac dragons, I only have Leo. It's level 9 cause I find him expensive to feed for the limited times I'd use him for battles. But when I do, he usually wins. Reading posts here, it seems that for any given dragon, it may be great in battles for you and another player has a dud. I read alot about this being the case with Dark dragon. So you may want to raise it to a 7,8,9 depending on food needed and try that in non tourny battles when zodiac is in use list and see how yours fared, then decide if it's worth feeding more.
But definitely get it to level 4 to breed. Then you can use it to try for the easier hybrids like crusader or quetzal. But with added zodiac colors it will be harder to get those with longer fail times than using pure diamond, but its bettr than no tries, I think.
Your other part of question about raising to level 15... for my tourneys I have only 3 dragons to level 15...diamond,metal and glasswing (who isn't a great fighter for me so I fed her in hopes). My other used alot dragons are mostly 14 with a few 13s and 12s. Huntress is a 13 and fights pretty well at that level but I use cleric, Light Mother, Dread Captain more since they're my 14s. So you may want to feed to 13 or better 14 to start with to save some apples. I do have a bunch of champs to vary in battles so 14s work for me but if you have only huntress, you may eventually find it bettr to get to 15,
Good Luck and enjoy those new dragons