I like the deck, have some questions. I am currently out of my tribal phase, but my first tribe/deck was hydras so I have a fondness of them, never did Gruul I like a lot of the things you have going on
1. Why are you committing so many cards to not letting you hydras be blocked by fliers? Does your playgroup use an excessive amount of flying creatures / weenie tokens they chump block you with? You will have huge hydra creatures, why do you care if they want to block with their Dragons/Demons/Angels/Sphinx's and their creatures get killed by your giant monsters? To me, I think of forcing my opponents to block as a form of creature removal. Personally, instead of all those slots you have, I would much rather put in way more trample enablers, so they can't chump block with any creatures, not just fliers. I agree that YOU blocking fliers is tough, so things like
Sandwurm Convergence and
Chaosphere I get, but even then I think if one of my opponents was playing big scary fliers, I would just focus on attacking them with my huge hydras first to knock them out and/or force them to block with their less-big scary creatures.
2. Mana base? 33 lands seems waaay low for these expensive CMC creatures. I would figure 38 at an absolute minimum, and it looks like you don't have any artifacts and very few other sources that generate mana for you. I get you are relying an Kolthys to provide mana for you, but 1 colored mana on your 1st main phase, is that really enough to cast these creatures?
Then, you can afford a $400
Taiga and a $50
Wooded Foothills, but your running
Evolving Wilds, Terramorphic,
Kazandu Refuge, and
Rugged Highlands?? Ouch... very slow, enter tapped, no real abilities or ways to help you. I would look instead at cards like
Boseiju, Who Endures ,
Highland Forest (can be searched for),
Lair of the Hydra (on theme),
Nykthos, Shrine to Nyx,
Cragcrown Pathway,
Rockfall Vale,
Karn's Bastion,
Castle Garenbrig,
Yavimaya, Cradle of Growth (when you don't need red mana it lets your mountains produce green for you),
Yavimaya Hollow for protection, Racer's Ring (at least you can sac it later to draw a card), Skargg the Rage Pits (to give trample),
Copperline Gorge,
Spire Garden,
Grove of the Burnwillows (who cares if they gain 1 life, you will be hitting them for 20 with your big baddies),
Bonders' Enclave,
Cavern of Souls if that's your thing.
As far as your mana producers go, I think
Karametra's Acolyte will be an all-star in the deck, after you get around the fact it costs 4 mana to get into play to begin with. The rest I don't really get tbh. It seems likely you want your Domri's producing mana for you, and that's fine I guess if PW's are your thing. Jiang? You want your huge hydra creatures being used to tap for a single mana? Vivien seems slow and clunky. How often are you passing your turn holding up 5+ mana to flash in a decent hydra?
Bloom Tender, a $20 card to produce 2 mana, 1 green and 1 red, on a 1/1 body that dies to anything? I would recommend
Springbloom Druid (lands are much harder to remove than creatures),
Rampant Rejuvenator (on theme), Auger of Autumn can replace Vivien with a similar effect and help you find lands,
Fertilid fits the +1 counter subtheme,
Gyre Sage fits +1 subtheme and should evolve a lot,
Somberwald Sage for casting your big creatures.
3. I'm seeing a VERY low amount of card draw/card advantage in this deck. You look like you would be top decking almost immediately. There are some very good green staples that you should be able to take advantage of with your Hydras.
Garruk's Uprising a MUST,
Kodama of the West Tree,
Greater Good,
Momentous Fall (paired with
Lifeblood Hydra is insane), swap
Kavu Lair for
Colossal Majesty or
Elemental Bond so only you benefit from the draw instead of letting your opponents score too,
Glorious Sunrise,
Inspiring Call,
Return of the Wildspeaker,
Zendikar Resurgent.
4. General Good Stuffs that could help:
Maybe best green card in MTG outside
Gaea's Cradle -
The Great HengeElementalist's PaletteAstral CornucopiaDoubling SeasonPrimal Vigor (cheaper version of
doubling season, but effects all players)
Hardened ScalesOverwhelming StampedeThe OzolithRhona's Monument
Mage SlayerBoxing RingBranching EvolutionUnnatural GrowthAsceticismFires of YavimayaMana BloomPrimal RageShadow in the WarpMaster ChefDeath's PresenceAnyway, those are my (likely unwarranted haha) thoughts. Maybe in your playgroup a lot of the cards you have in matter a lot more than they would in mine. I like a lot of your ideas though, might be time I think about going back to a Tribe for some Gruul fun!