The Paladin deck in the Kel’Thuzad class challenge is focused on playing the Echoing Ooze and buffing it before the end of turn to get two powerful minions. You can also get some nice combos if you give the Echoing Ooze divine shield and then play Blood Knight the next turn. The Paladin hero ability is a good counter to Kel’Thuzad’s hero ability after you break his armor.
The Echoing Ooze is your main way to win the game. Your deck has fairly bad late game (and in general) cards so you need to rush down Kel’Thuzad as quickly as possible. Play the Echoing Ooze together with buffing cards to gain two very powerful minions. One of the best combos is to give the Echoing Ooze Divine Shield and then play Blood Knight the next turn to get a 9/9. This is the way you can win the game, if you don’t use this combo you are unlikely to win the game.
Once you break Kel’Thuzad’s armor his hero ability will change to taking control of a random enemy minion until the end of turn. He is fairly bad at getting value with the hero ability but nevertheless you have to watch out for it. Don’t play powerful minions whilst your board is empty or only has one or two minions on it. Use your hero ability to summon a 1/1 as often as possible in preparation for the changed hero ability.