After playing so many times against boss animals, I've come with some theory that still need to be proved:
-Ultra Rare animals has a VERY slight chance of winning against bosses
-Gold and Silvers has a slight chance of winning, much better than Ultras
-Bronzes got the 40%-60% chance of beating the crap out of the bosses (my lil bro pwned Siegfried, Scorch, Atlas, and Galleon by using a 7000-8000 and less strength Icarus)
-Common cards has an extremely random chance of beating bosses, but it's up to the luck of the roulette
It's up to you guys if you wanna follow this, but I highly advice you to prove it yourself, since this theory needs to be reworked...
~ Haru Asakura