Oh, right. Gotcha now.
Yeah, that would be a bit of an issue. However, they're also a LOT more durable, so... it's a trade off. Whichever you prefer.
And a whole lot slower. If we do a turn count, by turn 5, which most cremation rushes will have pretty much won by, a fire master will do 0, 2, 4, 6, 8, for a total of 20. While a lava destroyer will do 7, 9, 11, 13, 15 for a total of 55, over double the damage.
You are better off with minor phoenixes in there place. Costs the same to get the same attack, plus, they are amazing immo fodder.