Cheap Kael'thas Sunstrider Priest Deck for Trial by Felfire Challenge

This deck aims to play very passively, using massive amounts of healing to overcome the boss's Turn 8 burst damage. During the early turns, you should play extra passive, only playing your minions to remove spells as necessary. On Turn 7, the turn before the boss uses 2 Pyroblasts on your face, you should ideally play down a Sandhoof Waterbearer and Lightwell to frontload some of your healing. After you have received the damage, you should ideally follow up with Grave Rune and Psyche Split on your Sandhoof Waterbearer to ensure you have a board and healing for the rest of the game. Alternatively you can use Power Infusion to put your minion at 11 Health and out of Pyroblast range or you can use Apotheosis to gain some additional Healing. If you are able to maintain a board with any minions that heal you for free, you can then comfortably grind down the boss while outhealing any future damage it does.