There is no good "combo" for getting a crystal unicorn. Every combo has the same chance for getting it. Only thing is that you have to use 4 different elements (from the 5 element that is available now, including dark).
But there are some tips which may be useful:
- Use low lvl (lvl4) common animals. Higher lvl animals, rares and super rares produce more coins, leave them in their habitats earning money.
- Choose elements depending on which animals you are missing (on possible fails). For example, if you have all red-green animals, do not use red AND green element, leave out one of them.
- If you use water and red on different sides, you may fail on Skyger. If you want to avoid Skyger fails (because you dont have the money for its hab) avoid red OR water. (Theoretically, if you use water and red on the same side, you may not fail on Skyger. However there is no water-red combo animal in Fantasy Forest )