@madking I wouldn’t say Sheggra’s mandatory (thankfully!). My dragon line-up consists of the 4 troops you’ve listed (in a different order, though) and I’m loving it!
Venoxia is the only dragon I ever obtained from opening chests (never had much luck with those ^^), so I don’t have a choice: these are the only 4 dragons I have in my collection. But beyond that, it’s just a very solid line-up, and it fits my play style like a glove.
Emperina is criminally underrated in my opinion. It’s one of my all-time favorite troops. Unlike @utfanx2, I don’t use her as a mere first-aid tent. I use her spell more liberally; it doesn’t just refill someone to full life, it also gives them extra life and attack. With the new life gain from Celestasia and attack gain from Venoxia herself, it all adds up very quickly. I wouldn’t use a different line-up even if I had Sheggra, honestly.
What I love the most about this combo is that you don’t need to spam gems and rely on looping turns (it never hurts, but it’s not the name of the game as with the old Venoxia + WebSpinner combo). You need to be very careful not to get Venoxia hurt too much before she starts getting tanky, but beyond that, you reach a point where she one-shots anyone with simple skulls.
Bone Dragon is just a safety net in this line-up (and also a 4th dragon, as required to get the magic bonus). In many games, he doesn’t even get to play, but I like having him handy when the stars are aligned. I have him at the bottom, because he’s the least of my priorities in this set-up, in terms of both mana and life (bottom is the second most dangerous slot to be in after top; I don’t want my precious supports, Emp & Celest in this case, to be in either slot, ever). Typically, I’m comfortable using his spell only when Veno has a lot of attack and life, when there’s already a fair amount of skulls on the board, and when at least one enemy has a lot of armor left. It can backfire horribly, otherwise.
Last but not least, Bone Drag usually gets a lot of mana from when Celestasia picks Purple instead of Green from Venoxia. In the double-Venoxia, Celestasia, Sheggra line-up described above, you’re setting up the board for the opponent and not gaining much for yourself when Celestasia happens to spam Purple. Doesn’t seem as reliable to me.
Shadow Dragon is great and he also has purple synergy, but on the other hand, he removes all yellow when he plays, which is bad for Celestasia.