Mulligans
General Mulligans
Prismatic Lens is a must-have in this deck and is a top priority card to find, but it is almost as important to have a low-cost minion to defuse pressure and allow you to combo off. Lesser Pearl Spellstone is also good, but relies on other cards and it is preferable to try and mulligan to lens or a low-cost minion. In every single strategy prioritize Prismatic Lens.Aggro Mulligans
Humility and Equality will help defend against attacking creatures and Hammer of Wrath can get rid of some of the more powerful minions while still helping find your combo pieces.
Midrange Mulligans
You should treat Midrange as a combination of the Aggro and Control strategies. Prince Taldaram can also get good value off of Warlock Midrange by copying powerful demons without their downside.
Combo Mulligans
You should usually be able to get the cards necessary when playing against Combo, hidden wisdom is good against some repeated combos and hammer of wrath can deal with some minion based Combos.
Control Mulligans
Control can deal with most of your big minions and the strategy would be to finish The Last Kaleidosaur and give it "can't be targeted by spells or hero powers" on one of its adaptations. Glass knight might also be useful against Priest in particular because it doesn't get hit by Shadow Word: Pain or Shadow Word: Death.
This is my first attempt at a deck surrounding the combo of Prismatic Lens, high-cost minions, and low-cost spells.
This deck is built around finding Prismatic Lens and getting a powerful card for a low cost. Dig until you find Prismatic Lens and protect it using Prince Taldaram and The Glass Knight. Get high value Dream and Death Knight cards to keep up pressure and win.
Stages of Gameplay:
Early-Game: Going second is always preferable as it increases card consistency(due to the 4 card mulligan) and doesn’t mess up tempo as you should try to coin-Prismatic Lens anyway leading to you always lensing before your opponents 4th turn. You should usually mulligan away any non-Prismatic Lens spells and high-cost minions so you can have a higher chance to get a Prismatic Lens. Keep any low-cost minions, such as Prince Taldaram and The Glass Knight, to protect your combo and to prevent accidentally lensing them later. Use removal and Silver Hand Recruits to defend yourself. Keep enemy threats at bay just long enough to survive and activate your combo. Play your low cost minions to defend yourself and your more powerful minions once you get the combo off.
Mid-Game: At this point you should desperately dig for Prismatic Lens if you don’t already have it, as it is needed to trigger the combo. If you are unable to find it, try to buff up your low-cost minions, remove the enemy minions, and even heal yourself if necessary to stay alive and defuse the board until you get a Prismatic Lens or enough mana to play your high-cost minions. In the unlikely event you Prismatic Lens into a low-cost minion, this strategy also applies. Try to defuse pressure as much as possible to make it available to create pressure late-game.
Late-Game: You should be able to have pulled off your combo and gotten powerful cards by this point, now its time to play all those leftover giant minions in your hand. get out your powerful minions and buff them with whatever spells remain, gain high-value cards from Ysera and The Lich King, build pressure and overwhelm your opponents. Always keep a board wipe handy in games against decks that can build up a presence very quickly, such as Cubelock or Taunt Druid. Prioritize large minions and powerful effects over board wipes and diffusion and out value your opponents with your large menagerie of Legendary minions and devastating spells.
Substitutions:
You can really substitute any of the big legendaries with big minions of any rarity, although Ysera and The Lich King should be highly prioritizes when building this deck.
Prince Taldaram and The Glass Knight are not esential but can heavily assist you in gameplay, when considering replacements, think about taunt and poisonous.
The only really essential part of this deck is Prismatic Lens, but try to keep roughly the same ratios and mana cost when deciding what to change.