There should be another way to fusion. To get to shiva you have one way where you are given many choices like light anu or water victoria, which can be nice additions to your team. But on the other side you have to catch 16 boltwings, making sure 1 is a fire one at least. So I wanted to suggest that it should be added another fusion table to get the fire boltwing, it won't be easy, but also not as difficult as it already is.
It could add some new astromons that could be great, like cura, cupid, succubus, tiger or something like those. As well as some low star ones like monkiki, bron, bellpup or shellie. And make them fuse in order to get a fire boltwing. As you can see for the astromons I mentioned, it would not be easy and you would need or high level, or high ammount of astrogems to get those astromons.