To breed a Level Exclusive you need to pair two dragons which
have all the elements of the target Level Exclusive between them. Legend counts as a wildcard , so it can be used for any of the elements. But at least one element needs to come from each parent.
The other requirement is that both of the parents need to be greater than or equal to the required level. This is the level claimed in the Build Menu. On the Facebook app this is a white number in a black circle. On Android it’s the number on the red flag.
If you do use a Legend for one of the parents, it bypasses the level
requirement though! Still, it’s usually better to not use a Legend to
breed Level Exclusives, since it makes for very low odds given all the possible dragons when Legends are involved.