Pacifism dose.

Also if you go against very slow decks that have a lot of big creatures sideboard: Dismiss into Dream and Glimpse the Sun God . (I'm not sure if Dismiss into Dream is legal in standard)

Kiora_Fan-01, Pacifism does boost Ethereal Armor because it's an enchantment that you control. It may be on the opponent's battlefield, but it's still under your control. For example an opponent in a game of sheriff decides to Dragon Mantle : Medomai the Ageless , only the person that casted the enchantment can activate the ability of Dragon Mantle

Launch the Fleet would be a nice card to run also.

First problem. Too many Heroics. Any more than 12 and you're honestly starting to drown your deck and when 30% of your deck is cards with Heroic you really need to reconsider whether they are necessary. Ephara's Enlightenment could be a good choice if you want a lot of creatures out there stacking counters. Mizzium Skin is a great card, Detention Sphere or Banishing Light are great removal cards your deck could use, Ethereal Armor and Aqueous Form are great cheap enchantments.

You already run Hidden Strings and Phalanx Leader so if you add Nivmagus Elemental it makes a great combo. When you cipher Hidden Strings onto a creature you can target two heroic creatures - most ideal is Phalanx Leader - to give +1/+1 counters, then use Nivmagus Elemental to exile the copy Hidden Strings . With two Phalanx Leader the Nivmagus Elemental will get 4 +1/+1 counters and all other creatures will get 2 +1/+1 counters. You can also combo Nivmagus Elemental with the 1 mana cost instant spells by targeting Phalanx Leader and exile the instant with Nivmagus Elemental to get that extra damage to win. Speaking of 1 mana cost instant spells I'm a big fan of Triton Tactics and Retraction Helix because Triton Tactics lets you target two creatures, untap them, give them +0/+3 and minions blocked by it don't untap. Retraction Helix can do quite a few things like trigger heroic, return a creature that is enchanted or a blocking minion letting your encoded minion attack directly and trigger the cipher of Hidden Strings .

I really think you should replace the 4 Akroan Skyguard with 3 or 4 Nivmagus Elemental becuase Phalanx Leader plus Nivmagus Elemental with Hidden Strings can win games because you can get lots of +1/+1 counters fast.

I strongly suggest you check out Heroic Leader Tournament Deck by I_H8_U_M8 as he has experience playing with this deck and has figured out a way to do 14 damage with Nivmagus Elemental and 6 with Phalanx Leader by turn 3.

Razorclaws, I understand where you're going. The Hidden Strings target but they don't have to resolve. So then the Nivmagus Elemental will benefit for the most.

This looks a lot like my Hidden Strings Heroic deck. So of course + 1I would switch out the Phalanx Leader that double white is a killer. Battlewise Hoplite is just a more consistent card. 4x Gridlock is a little bit of overkill I think. Plus you want another Island at least.

+1 from me, looks decent

And I agree with BrandonJamesCAC that you should replace Phalanx Leader with probably Battlewise Hoplite . Not only the cost, but you don't have that many creatures to make him valuable.

I always liked Artisan of Forms as a 2 of in those kind of decks. Very often, it's not only enjoyable but also game changing if you can copy the opponents biggest threats.

Artisan of Forms is definitely another good option. The U/W Heroic decks are tough to get right. I am nowhere near being finished. Obviously if you get a Hidden Strings on a Fabled Hero in a good spot, you're going to do well. The problem is drawing hands that look like a magic show, no pun intended. Nothing more useless than a bunch of bullets and no gun