I haven't really farmed T50 with a fire deck, I personally prefer using a shrieker rush. Although a fire deck can be faster, there's more clicking involved and the phoenix/ash cremation animation flashes the red light in your hand. Fire creatures also have less hp, and die more easily.
I'm not sure about the explosions as a must have card (but then again, I haven't farmed much with a fire deck so don't trust me on this). There's no perm-control in the shrieker deck and I do fine with that, but the earth deck has the advantage of burrowing.
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The gavel can give you 3-turn wins, but you must not go first and start with 8 cards.
Starting hand:
2 towers
2 cremations
1 phoenix
1 lava destroyer
1 dragon
1 gavel
1st turn: Play both towers, double cremate the phoenix, play rest of hand. Damage = 29
2nd turn: Draw and play phoenix card, growth lava destroyer. Damage = 29 + 4 + 2 = 35
3rd turn: Growth lava destroyer. Damage = 35 + 2 = 37
Total damage = 101
Variations:
Draw lava destroyer 2nd or 3rd turn and play on 3rd turn*.
Total damage = 29+31+40 = 100
*Gavel takes 2 random quanta to play, if it uses your last fire quantum first turn then you need to draw a fire tower with your lava destroyer as your 2nd and 3rd card.
Personally I prefer phoenixes to cremate because you can double cremate them, or if it isn't necessary then you can revive them next turn for 4 damage. However, you need more burning towers to run this deck, especially if you are aiming for 4 turn wins as most of them start off with being able to cremate a phoenix on the first turn with 2 towers in play. Probably 8 towers is enough instead of 9 even.
I am no statistician, but the chance of the 3-turn win would be extremely low, compounded by the fact you must not go first and must start off with those 8 cards. There may be other ways to win in 3 turns but I haven't thought of them for this deck. Putting in the gavel however can also be a disadvantage if rainbow decks have a steal, your chances of EM goes down a little.
But all said, cremation decks work very well vs. T50, probably better than a shrieker rush even, but I'll probably stubbornly stick to the shrieker deck for the reasons mentioned above.