Managed this with:
Blighthawk H/H - xxx
Unborn Valk H/H - x,2,2
Infected Squirrel P - 2,2,x
Blighthawk is just to soak the swaps so start with that in first slot to get Valk in.
Swap Blight back in, so it gets swapped for squirrel
squirrel dies to swap but get blighthawk in anyway
ghostly bite kills Gloamwing and despite your hawk also dying it seems to get credit for achievement
Infected Squirrel comes in
Infected Squirrel dies in the swap
Bone Serpent comes in
Ghostly Skull is swapped in
Haven't tested this with other breeds but the squirrel has to have more HP than the Serpent.
This is a strat that was born out of desperation. While I have the pets mentioned above and in the comments, I don't have them leveled. ;>.>
Eye of the Legion (P/S) 2,2,1
Zomstrok (P/H) 2,2,2
Infected Squirrel (B/P) 2,2,1
Pass. Zomstrok comes in.
Pass. Zomstrok dies here. Infected Squirrel comes in.
Infected Squirrel tends to die here. Either way, Eye of the Legion gets pulled in.
Gloamwing goes down! :D
I will say that this dependent on RNG. First time I tried it, I.S. lived. Second time, E.o.t.L. had about 100 hp left. Was a close one. I also understand that E.o.t.L. is a tcg pet. Sorry about that. >.<
Ghastly Kid 2/2/2
Widget the Departed 1/2/1
Infected Squirrel 2/-/-
Turn 1 - Ethereal
Turn 2 - Haunt
Turn 3 - Stampede
Turn 4 - Gloamwing brings in Widget
Widget the Departed
Turn 5 - Call Darkness
Turn 6 - Spectral Strike
Turn 7 - Pounce
Turn 8 - *(If widget doesn't finish the job, either of the other 2 pets will because they will both be alive)