Tracking - Helps you to find whatever you need when facing different classes. Best at opening hand.
King's Elekk - Cheap card draw and a 3/2 body. Also helps you know what type of deck your opponent is playing, helping you decide what to keep in Tracking.
Wild Pyromancer, Explosive Trap, Powershot - Early game AOE that help you fight against aggro decks.
Hunter's Mark, Big Game Hunter, Deadly Shot, Explosive Shot - Late game removals that help you fight against control decks.
Sen'jin Shieldmasta, Sludge Belcher, Antique Healbot, Alexstrasza - Taunts and heals that enables you to survive, especially after Majordomo's deathrattle triggers.
Majordomo Executus - The main reason you play this deck. This is the main way to win when facing other control decks. Although 8 health may be a dangerous situation, but remember that Alexstrasza can set Ragnaros's Health to 15. You will certainly have fun using pyroblast every turn.
Feign Death - A combo card that works well with Sylvanas Windrunner, Dr. Boom, Piloted Shredder and Savannah Highmane. Sometimes also used with Majordomo Executus to deal the last damage you need. Feign Death + Sylvanas Windrunner = You win this one, friend.
Gahz'rilla - Your third winning condition other than Feign Death and Majordomo Executus. Combo with Wild Pyromancer and Powershot, sometimes even Explosive Shot on an adjacent minion.
Always mulligan for Tracking and King's Elekk whatever you're facing. And other cards that you may keep are:
VS Hunter and Paladin: Wild Pyromancer, Explosive Trap, Powershot, Sen'jin Shieldmasta, and if you have AOE already, keep Flare to remove secrets.
VS Warrior: Assume that your opponent is a control warrior, because this deck can't beat patron warrior XD. Keep Deadly Shot, Piloted Shredder. If both Feign Death and Sylvanas Windrunner appear at the opening hand, keep them.
VS Warlock: It depends on whether you think they are zoo or handlock. If zoo, keep AOE cards; otherwise keep nothing but King's Elekk and Big Game Hunter.
VS Priest: Deadly Shot and Piloted Shredder
VS Mage: Assume they are mech mage. Keep any 2 drop, like Wild Pyromancer and King's Elekk, and removals like Quick Shot.
VS Druid: Deadly Shot and Quick Shot.