In general, the deck wants a fast start to pressure the board early. As such, low cost minions--especially those that tutor for or accelerate secrets--are prioritized in the mulligans. Usually, secrets should not be kept in the opening but there are some exceptions with secrets crucial to specific matchups when Kirin Tor Mage is already in hand.
Aside from Aggro Shaman, which the deck can often outpace, Aggro matchups are typically unfavored. Pirate Warrior and Egg Druid both do a lot to disrupt the primary goal of early board presence, so finding a good start in the mulligan is critical.
Having a Turn 1 Mana Wyrm is even more important than usual. Frostbolt should be kept, but not necessarily played on Turn 2 in every situation. Coin + Mad Scientist into Medivh's Valet can neutralize early minion pressure while gaining board initiative. However, Valet is strictly worse than Arcanologist and Mad Scientist (or even Frostbolt in some cases) on turn 2 in other cases and should not be kept without some degree of certainty that a secret can be accelerated into play.
Combo MulligansGenerally, combo decks are a favorable matchup because they tend to do very little in the early game. The exception, however, is Combo Priest which does a lot to disrupt early turns with overstated minions. Here, Primordial Glyph is a good keep because of it's ability to find Polymorph or other efficient removal. Other combo decks play a lot like Control matchups in that you want a quick opener and an Ethereal Arcanist that goes unchecked can win the game.
As usual, the goal is to pressure with minions early, accelerate secrets into play, and finish with burn spells. Against Control decks, however, Ethereal Arcanist really shines and may be worth a keep--should the rest of your hand be strong enough, of course). If you do play Arcanist on Turn 4, you really want to have a Counterspell behind it to avoid early removal spells from your opponent and another secret in hand (or play) to keep it growing.
Midrange decks that don't go very wide on the board tend to be good matchups. Here, pushing the tempo early is the path to victory, but Frostbolt and Medivh's Valet can do a lot to negate opponents' attempts at gaining control of the board. As with Aggro matchups, Medivh's Valet becomes a less valuable keep without the ability to cheat a secret into play early.
Secret Mage has been an archetype that’s been on the fringe of the meta for some time. In the past, it was seen as a gimmicky deck that was strictly worse than the Flamewaker Tempo builds.
With Flamewaker rotating of out of Standard and Journey to Un’goro bringing Arcanologist, however, many players began revisiting Secret Mage and found that several of the tools introduced in One Night in Karazhan and Mean Streets of Gadgetzan performed better than expected. Playtesting in Standard had carryover into Wild where the deck becomes even stronger, just on the border of Tier 1, thanks to the availability of cards like Mad Scientist and Duplicate.
In general, the deck relies on pushing the tempo by developing early minion pressure, accelerating secrets into play with Mad Scientist and Kirin Tor Mage, capitalizing on secret synergies, and closing the game with burn spells.
Mana Wyrm: The best 1-drop available to Mage and a card that can get out of hand very quickly making it crucial for getting ahead early in the game. Cheating secrets into play with Kirin Tor Mage offer an opportunity to further ramp the Wrym.
Arcanologist: This card replaces itself with a secret, thins your decks improving future draw, and ensures a secret can be in play on turn 3 all on a well-statted body. Arcanologist should always be played on Turn 2 play when Kirin Tor Mage is in hand.
Mad Scientist: Provides a similar effect to Arcanologist but puts the secret into play instead of replacing itself. On paper, this makes it a strictly but it does have some drawbacks, including the understatted body. Usually a better Turn 2 play than Arcanologist when Medivh’s Valet and Kirin Tor Mage is not.
Medivh's Valet: With a secret in play, this card provides a massive tempo swing or the last few points of burn needed to find the kill.
Counterspell: Whereas most other cards generate tempo directly, Counterspell does so indirectly by negating as much as an entire turn for your opponent. Timing is crucial with Counterspell, so consider what high value spells your opponent’s deck has and when they will be play.
Duplicate: A critical hand refill mechanic, Duplicate has very few bad targets in the deck. While Duplicating a 0 cost Kabal Crystal Runner is great, the real purpose of Duplicate is to recover from a board clear, which the deck is susceptible to in the early game.
Ice Block: Ice Block provides very little in terms of tempo generation but is the one secret that can be counted on the stay up throughout the game, ensuring value from Medivh’s Valet and Ethereal Arcanist. The extra turn is occasionally relevant, offering the option to set up a two turn lethal.
Kirin Tor Mage: An aggressively statted minion that provides one of the methods of cheating secrets into play to ramp Mana Wrym or reduce the cost of Kabal Crytal runner. Kirin Tor Mage is the deck’s best Turn 3 play and a great Duplicate target with additional secrets in hand.
Ethereal Arcanist: With no secret in play, this card is terrible. The upside, however, against slower decks makes it worth the inclusion. Protect this minion with a secret in play for a few turns and win the game.
Kabal Crystal Runner: A card that benefits greatly from all of the secret acceleration. At worst, it’s usually a 5/5 body on Turn 4. At best, you get a handful of 0-cost 5/5’s.
Kabal Lackey: While Lackey does cheat secrets into play like Kirin Tor Mage, it does so a little too early to get good value and on a body that you hate to Duplicate. Finds a spot in some Standard lists, but does performs poorly in Wild.
Arcane Intellect: Typically, tempo oriented mage decks such as this need the extra card draw from Arcane Intellect. Here, however, Duplicate and Arcanologist provide more controlled hand refill and the secret tutoring thin the deck and improve top decks in the late game.
Effigy: A card that performed surprisingly well in play testing and can be rotated in depending on matchups. As mentioned, the deck suffers against board clearing and Effigy can protect against that.
Piloted Shredder: Piloted Shredder is a card that barely missed the cut and does well in many situations, but not quite as well as Ethereal Arcanists. Should be played in place of Arcanists, however, in lists that are not running Ice Block.
Firelands Portal: Like Medivh’s Valet, Portal provides a good tempo swing, but does so a little too late in the game. By turn seven, you really want to be finding ways to close the game rather than fighting for the board.
Played optimally, Secret Mage has three distinct phases: Early Board Pressure, Secret Synergies, and Closing Burn.
Early Board Pressure (Turns 1-3)
In the beginning of the game, you want to snowball the board into early chip damage to your opponent. A 1-2-3 curve of Mana Wrym, Arcanologist, Kirin Tor Mage is usually hard to beat if uninterrupted. With a secret already in hand, Frostbolting a minion to protect Mana Wyrm is usually a better play than Arcanologist.
Secret Synergies (Turns 4-6)
With the board secured, you can begin taking advantage of the numerous secret synergies in the deck. With a secret in play, Ethereal Arcanist on turn 4 creates a remove or lose scenario in most cases. If Duplicate is up, paying 4 mana for Kabal Crystal Runner on turn 4 is good, otherwise it may be better to wait until turn 5 if you can pair it with another secret from hand. Medivh’s Valet does a lot of work in these turns, often helping lock in your control of the board.
Closing Burn (Turns 7-8)
Burn isn’t always essential to winning games, but against slower decks you may need to concede the board in order to set up burn to close out the game. Knowing how much damage you have available from hand and where to direct that damage is critical to piloting the deck successfully.
Thanks for reading! Win rates and statistics for the deck can be found on my personal site.