The task condition is too general; there are many possible solutions. It's unclear whether they are in the same stack, what spells can be used, and what spells are prohibited. For example, you could use Aotd to slow down the dragons, contain them with a force field, and with a million mana, cast either Berserk, Explosion, or kite them with a wall of fire, and so on.

Yes, you need to come up with a specific solution. If you claim "a million mana," then provide more details on how you obtained it (through possible gameplay methods in Heroes 3). The task is for Heroes 3, so you need to consider all possible game situations and find the one that provides a solution. The only clarification: we are facing a single stack of 9999 crystals. You can set everything else as you wish. However, describe your actions more specifically; if you write "a million mana," explain how you obtained it, whether that "million" is enough for everything, and provide calculations (I'll give you a hint: you'll have to do them). The task has a solution.