Built this deck recently and used it IN Legend rank. Does surprisingly well against the frequently played mid-range hunter
Mulligan (universal) : Mulligan for 1/2 drop minions. Throw away spells
Strategy : In my opinion, Secret Mage is a deck that tests your opponent's ability to keep up with you. If he/she can't keep up, it is easy to secure a win in the early or mid game. Try to play on a perfect curve every turn, especially during the first few turns. Also, I must emphasize it is very important to maintain board control as Secret Mage decks rely on board control + burn spells to secure a quick win.
Notes : Remember not to play Potion of Polymorph into Mirror Entity as you will end up with a sheep. Correct order is Mirror Entity into Potion of Polymorph
Rather than playing the usual run-of-the-mill Mirror Entity - Counterspell Secret Mage, it would be better to have more secrets to increase your chances of drawing them and strengthening the synergies more
I did not include Babbling Book as I find it to be weak-performing coupled with the fact that the spells you get are randomised so it would really suck if you get spells like Shatter. I feel that Babbling Book may be nice to have in control mage decks, as control mage decks are less aggro as compared to Secret Mage which is more of an aggro deck. I would rather draw a Bonemare on my late game turns than a Babbling Book, so...