Well with the deck being larger and I'm assuming modern casual, it would probably benefit more from being a larger deck. Yeah I understand the draw probabilities and what not but he'd be able to fit more mana into the deck (meaning more plays per turn), he'd be able to pay more costs of creatures effects (such as
apocalypse hydra) and since he's running three colors it'd give him some more room to work in various plays that an opponent would not expect (such as
aerial maneuver), can you imagine a flying 10/10 hydra with first strike and trample? But looking at this deck overall there are a few cards that I would suggest removing or simply trading out. Mainly the ones that have defender. Reason being, hydras are highly aggressive creatures, if you play a strong and fast hydra in a slow, defensive deck then you won't see the greater effect of that creature being put to use. If you look at the Selesnya and the Gruul guilds from the return to ravnica block then you may find the other cards that you need to strengthen, speed up and solidify this deck.
Also thank you wwolfe for pointing out that
steely resolve. Its' effect is rock solid but for a deck like this I'd recommend side boarding it so you can use the combat ticks. If you prefer to keep it in the deck then you'll need to pay a lot of mana to make your creatures grotesquely large when they enter the field. In exchange for the enchantment would you consider
oak street innkeeper? It basically says that: all your tapped hydras that you just attacked with cannot be targeted by YOUR OPPONENT. This still lets you play cards such as
burst of strength or
aim high when you need to.