Sunderflock

Once you determine the cost to cast Sunderflock, you may activate mana abilities to pay that cost. If the greatest mana value among Elementals you control changes while activating mana abilities (probably because you sacrificed one or more Elementals), the cost to cast Sunderflock remains what you previously determined.

If an Elemental on the battlefield has X generic mana in its mana cost, X is 0 for the purpose of determining its mana value.

Once you announce you're casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can't try to remove Elementals you control from the battlefield at that time.