Im having alot of trouble with this and I can only use legends…
What’s your strongest Legendary Amphibian? Just want to know this one little detail before proceeding…
A level 20 ichthyostega
Ok. So here is my suggestion.
Put your Pterodactylus in first spot. Opponent will switch to Segnosaurus. But be careful, in your first move block just in case he doesn’t switch though it’s unlikely. Even if he does switch, Segnosaurus can knock out your Pterodactylus in one shot so be careful.
Put your Concavenator in second spot. Once you’re done building reserves in with Pterodactylus, have the Concavenator knock out Segnosaurus, and also try to knock out one of the other carnivores with it too.
Rather than Ichthyostega I would suggest Baryonyx in third place since it has higher health and probably higher attack than Ichthyostega with advantage.
Let me know if it helped. 
I would put maxed Diplocaulus in first spot, he will switch in segno, block as much as you can. 5 or 7 bonus are ensured. Use Acrocantho and try to snatch out one of those carnis. Use your biggest amphibian to take out the other two. Best you dan do is this
I love how Diplocaulus was promoted to Legendary. 
His strongest is a level 20 Ichthyostega as he mentioned, even with advantage it’s weaker than his Baryonyx and other VIPs.
Thanks I won the battle!
You’re most welcome. Glad I could help.
Good luck. Hope you get a Locked Creature like Suchomimus. 
