I never know if I should share my successes (more like luck) as I don't want anyone else to feel discouraged, but I do share in hopes that maybe I have info that will help someone else. When I woke up this morning, I had things (including a Seamur) waiting to be hatched and an animal ready to come out of the evolution flower. I bought a small boost and got to work. I didn't keep an exact count, but I used approximately 200 gems doing speed breeding for the Wendiglow. This was on my baby account, which only has a limited number of animals to work with, so it wasn't quite so overwhelming. I bred Skyger with each of my dark/water animals one by one with no luck. Then I started down the list using Frostfang as the main parent, and finally got Wendiglow using Frostfang and Chameneon. While I had my boost activated, I also fed my Seamur the 50 times and then fed the Wendiglow up to level 10, so now I have completed the Leaderboard milestones (I'm still not a contender to get the deer, though), completed the Wendiglow goal, almost completed the Scorpio goal, and now I am at 94% for my Witch's Familiar and 97% for my Sorcerer's Steed. I also have 15 bombs ready to find me some runes! Unfortunately, I'm sure that I earned more than 15 bombs during my speed breeding, but I didn't take time to use them before I went over the limit, so I probably missed out on a number of runes, too.
My main account was unable to obtain the Candy Horn or the Chihuacabra, so there is really no point in trying for the Wendiglow there. I am only at 34% on treats but 97% on Tricks. I'm not sure if it's worth using an entire habitat space for just one animal (Sorcerer's Steed) since I don't have any other spooky animals on that account, unless I win the Jackallanturtle - whatever it is called.
So keep trying! You might get lucky. On Dragon Story I did a final "hail Mary" breeding for the Little Monster dragon and got it just in the nick of time. Not soon enough to gather the Treats for it, but at least I got a super cute little dragon out of it.