I see a lot I would want to do in this deck, but at the least consider Congregation at Dawn. But I like what you got pretty strong, especially against Grixis.
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I think your right about Congregation it's not giving you too much help other then just combo'ing certain creatures(Mostly just Pridemate with healer of the Pride). I still think Descendants' Path is great, mostly cause your going to be getting out 2 cat's rather then just 1 each turn, think of it as I get to draw 2 cards rather then 1. The problem with it is that if you play it you better have A LOT of cats to make it worth while cause you'll be skipping a lot of other cards. I think with what your doing 3 or so Descendants' is good but what you have right now is really great mana curve is beautiful the 2/1 White vs Green is modestly to remember. I really think 3 Descendants' is still worth your time cause of the speed factor in which you'll grow a force is twice as much. The only thing I can think of recently aside that is Loam Lion, Wild Nacatl and Sunpetal Grove are all reliant on a Forest or a Plains so that's something to remember. Other then that looks great...
A budget deck, I'll keep that in mind... If Mentors and Brimaz are coming out you might want to look at mana sourcing as well Sunpetal Grove is great cause you get the mana when it comes into play if you have a Forest or a Plains. But paying a colorless to get GW is kind of awkward and kind of pricey($ wise) So I wanted to toss out some land ideas for you to consider not that they're better in play value but in terms of price $ value they're about 1.50 cheaper The one I like the most is Veldt fits your theme pretty well, the list goes: Vivid Grove, Vivid Meadow, Veldt, Selesnya Gate, Saltcrusted Steppe. From your post I kind of see where you want to go with the deck and the minor in healing. Knowing that I would say maybe go with Healer of the Pride instead, mostly cause this gives you 2 life every time creatures come into play. You have a lot of token generation so +1/+1 counters on Pridemate will trigger a bit. I also enjoy your idea of Kemba's Skyguard and the mistake of Kemba, Kha Regent + Behemoth Sledge make a great combo with Pridemate & Token generation. If you are worried about fliers perhaps Shield of the Oversoul? I think another good replacement for Mirari's might be Collective Blessing instead of Cradle of Vitality. A few other things to think on might be Selesnya Charm for removal / Trample / Token or Reap What is Sown for counters and Rancor for Trample and return to hand effect. Another favorite of mine that might be good in this situation is Congregation at Dawn as you want to excel at aggro and creature combos. Congregation can also be a set up for Decedent's Path as Congregation is an Instant and you can do it at the end of your opponents turn.
I'm gunna go off on a tangine but its just food for thought, love the deck... Ajani's Pridemate and Ajani's Sunstriker are great and theme'y with Mentor of Heroes. With Mentor's ultimate if paired with Cradle of Vitality & / or Sunbond and Ajani's Pridemate things get dangerous. Sneaking a Heroes' Reunion or two wouldn't be bad ether being instant to save your life and act as a nasty giant growth if you will handing out 7 counters to Ajani's Pridemate, Cradle & / or Sunbond. Unflinching Courage or Behemoth Sledge could help capitalize on the huge counter gain with these cards even more so with trample and lifelink. Lastly, Felidar Sovereign can make Mentor of Heroes ultimate fatal depending on how you want to run your deck RP or cheesy. Things I feel are not so useful: Mirari's Wake, White Sun's Zenith... Mostly cause Mirari's only excels with White Sun the rest of your deck is on a 3 drop on avg and Mirari's isn't doing much work for you. Without Mirari's, White Sun's is rather under utilized since your deck is decent fast so its very dependent on Mirari's unless its the late game and you have a large mana pool. You also get cat token's from 2 other sources a lot cheaper and easier then from White Sun's.
Maybe a few Nephalia Drownyard's bout all I can think of but I like it
Just a thought of interest perhaps ' Sunglasses of Urza ' ?
I love lonely Sandbar in this its just so Apathetically cool... I like your artifact thing it is more you then the merfolk thing I found for recommendations. I still think you really need Traumatize, Memory Erosion, Tome Scour, Brain Freeze. If anything at least put 1 Traumatize cause that + Keenstone = instant win...
Solid, I guess my only itch is Inlaid Dagger. I like Blazing Torch to spam a lot more due to putting damage were ever you want. Pitchfork would also be something I would consider. I like Mausoleum Guard in this deck but at 4 mana that's kind of iffy. You might want to increase Mentor of the Meek too at least 3, I really wanted him out fast to make him effective in my green / white deck.
Need a ' Ulasht, the Hate Seed '
I was doing some research, You should ~:+:Highly:+:~ consider 'Core Prowler' for this deck.