No problem! The back sprite is the really tricky part. I’ve only ever been able to do it by modifying the front sprite to make it look like the back sprite; for instance:
Front Sprite, Perfect Kyurem
Back Sprite, Perfect Kyurem
Here I took the tail and head and mirrored them, then I combined them with a heavily altered backsprite of Zekrom. For the Primal Giratina sprite that you didn’t care for, I flipped it horizontally, made the portal the top layer and messed around with the head for awhile.
It really helps to use the box select tool (with the “Add” function so you don’t have to select it all in one click-and-drag motion) to separate pieces of the sprite into layers so you can fiddle around with their positions (legs, wings, different parts of the head, etc.)
In the end, it’s a lot of minor detail work in adding a pixel here, recoloring a group of pixels there until you end up with something you’re satisfied with. I have so many “work-in-progress” versions of a single sprite!
Hope that helps!