I'd suggest to upgrade the toadfishes too, if entropy makes a mono deck it will most likely contain antimatters. Puffer fishes pierce through antimatter with their poison. You get the point 
Also nice to counter Maxwells, all they can kill is the dragons! (hold the dragons till you're sure there is no AM btw, use of for the finishing blow)
Deck Options:
1. Mono water 30 ups Event card 3 for ups, or Event card 1, which allows vault styling
2. Valkyrie Event card 1, thus vault styling allowed
3. PU/Dim deck Event card 1, thus vault styling allowed
On the antimatter, you can also use the squids to freeze your own antimattered creatures

But upgrading the puffers is a nice move indeed.
On the other hand, entropy is a strong team, so I would prefer to use Valkyrie or PU/Dims, because we still have the other one left when Rav loses (always assume the worst, that means you can only do better

).
Basically we have to:
Build an fully upped water bow that has a nice win percentage.
Build a duo in such a way that is has a good chance to win, but still leaves us with another viable deck if it loses.
Then we have to test both options, and see what gives us the best win chances the round after this.