You have 20 turns to beat him and if you do not beat him on the first try, you have to spend more energy.
The amount of damage done carries on in the next try.
Match on the Gas Flames to ignite it before the Alpha.
The Green Death’s ability spawns 2x2 patches of gas. If gas is already present on the board when its ability is ready, the Green Death will ignite the gas, sending fireballs to your Dragons. Matching by or on the gas will ignite it, sending fireballs back at the Green Death