I suspected Exert would be a lot like Cohort, and perhaps Phasing: a sucky, lose-a-turn ability, and unless I am missing something, I think I was right. Here is the final, if you can call optimized, deck. I lost every game, and therefore, if I did any optimizing, it was to lose faster? I started with Naya (White/Red/Green) and then I very soon cut Red out. It didn't matter, this deck was born to lose.
Here is my track record, an incredible 0-19 record. This has to be the worst record of any deck I have ever optimized.
Here, I get board-wiped. My opponent was playing around Trespasser's Curse, something I want to do. I am surprised Trespasser's Curse is common, and I like that! It's always great to have a common that is more useful than for just Limited.
For the rest of this post, I am describing a loss against an Approach of the Second Sun deck. This description is more about my opponent's interesting and successful deck than about my terrible Exert deck. I still don't know why As Foretold is in this deck. I guess you can get some free-casting out of it (At the beginning of your upkeep, put a time counter on As Foretold. Once each turn, you may pay 0 rather than pay the mana cost for a spell you cast with converted mana cost X or less, where X is the number of time counters on As Foretold.) I did not see my opponent do much free-casting. They were able to build up the mana for hard-casting.
My Exert critters are doing "their thing" here. It's like pitting kittens against lions, though, and I am the one with an army of Exert kittens.
I get board-wiped.
Target removal at work.
My opponent got to play all four Anticipates in their deck.
They even got to use Disallow.
More target removal.
Here is the first Approach of the Second Sun (If Approach of the Second Sun was cast from your hand and you've cast another spell named Approach of the Second Sun this game, you win the game. Otherwise, put Approach of the Second Sun into its owner's library seventh from the top and you gain 7 life.) In this case, this is the first cast, and so it goes back to the deck.
I finally get to hardcast my big critter.
It gets removed.
And the game ends.