Something to consider about the deck: if you have Kalonian Hydra out and attacking, you've probably won the game. Therefore, all the other creatures that you include for counters aren't actually benefitting that much from it (although they benefit from Exava).
Also, the earliest you can field a decent threat is turn 3 (and [Deathrite shaman] isn't likely to give you any mana acceleration since you don't have any self-mill). You'll take quite a beating from aggro decks, so you might want to include a bit more creature kill/burn. You'll also be vulnerable to turn 4 board wipes, but with your mana acceleration it's quite possible you've already swung at least once with the Hydra.
Scavenging Ooze isn't too useful if you don't have creature cards in various graveyards, but it may be worth including if you put in more creature kill/burn.
Duress in the sideboard against control matchups is awesome. Just be careful not to cast it too early (if your biggest concern is a counter spell, cast it the turn before your big play. If your biggest concern is a board wipe, cast it the turn before they hit 4 mana, etc.)
Overall, I'd suggest considering Kalonian Hydra to be a big beatstick, not someone who doubles counters, and build your deck around that. Some cards are worth encouraging for their synergy with the hydra + being a good card, like Rakdos Cackler . Acceleration, creatures that make your hydra hasty (including Exava if you do end up using a few creatures with counters, like the cackler), and a bit of creature kill/burn and i reckon you've got a sweet deck.
Transguild Promenade My general impression of this card is the mana/turn lost to getting it untapped is not worth the trouble. It's probably worth cutting, unless you have huge mana problems (and of course, if you want to spend obscene money, Cavern of Souls is stupidly powerful.
another point- you can of course include other big beatsticks since you have a lot of acceleration! There's some 5 drop creature that is/was the standard five drop, can't remember what it is... Used to be Wolfir Silverheart , changed to something else, I think? Someone help me out?
You need removal, Personally i would say add flesh/blood because blood is a better version of pit fight, and if you get to fuse them you get to make your creatures bigger. Especially if corpsejack is on the field and deal alot of damage to your oppenent directly or to his creature.