Hmm, alright, I'll see if I can represent my thinking process properly
5
5|4 + flexible ability Thunderbird (it can do a lot of things with its ability xD), it will get flight as well
4
3|3 + fix ability Dive (dmg) Wyrm
3
3:2 + fix ability of mana generation Firefly
10
9|6 Azure Dragon
The higher end creatures seems to be Cost - 1 for their ATK, maybe that's the thing that's throwing me off xD
When compared to other elements, the thunderbird seems a bit more cost efficient on the damage per turn, survivability and flexibility of its ability. I wouldn't say it is absolutely OP, but it would definitely become a favoured contender. Abdomination at 5
, Flesh Spider at 3
3:3, Graviton Mercenary at 4
3|5, Blue Crawler at 3
, 3|3, etc..... seems less desirable in comparison.