Hi! I made this deck after the release of all the Naxx wings and have used it to get to Legend in the last 2 seasons (August and September). I have tried to make Secrets Mage work in the past but never had any luck however a number of Naxx minions have made this archetype viable.
Discard everything that isn't 1 or 2 mana. Discard Thalnos unless you also have an Undertaker. Kirin Tor + Secret are an exception to this rule if you have a turn 1 play + coin (live the Mana Wyrm -> Kirin Tor/Secret dream!)
The aim is to establish early game board control with a combination of Undertaker/Loot Hoarder/Mad Scientist/Mana Wyrm/Frost Bolt. Even if you don't have a relevant buff card always try and play one of your 1-drops on turn 1 and force your opponent to respond. It's important to apply pressure from as early as possible so even suboptimal combos like Mana Wyrm -> Loot Hoarder are desirable. Turn 3 is when you get a chance to play your first midgame cards with Kirin Tor + Secret and if you already have a board at this point you have a great position.
Kirin Tor, Water Elemental and Sludge Belcher are all really stat efficient minions and are the engines of your deck that consolidate your early game. Don't be afraid to play Kirin Tor Mages without a secret. Fireball is good removal here but try to avoid blowing your Polymorph here as it is doing double duty as your silence and hard removal so it best kept for things like Highmanes and Sylvanas. That said Poly on a Sludge Belcher to maintain board control and put life pressure on your opponent is a valid play. Harrison Jones and Azure Drake provide sources of draw with a decent body and potential utility.
At this point your big legendaries come into play. Both Sylvanas and Kel'thuzad excel when played into contested boards to gain board control, which is why they are in the deck. Flamestrike is similarly effective and you should feel to use it against small enemy boards to take or extend control. At this point in the game you are usually fishing for lethal so be very aware of damage from your hand with Fireballs, Frostbolts and Thalnos.
Replacements<pre>Bloodmage Thalnos -> Gnomish Inventor
Harrison Jones -> Azure Drake
Sylvanas -> Loatheb
Kel'thuzad -> Ragnaros or Loatheb </pre>