Dragonkin Berserker

Once you announce that you're activating a boast ability, no player may take actions until the ability has been paid for. Notably, opponents can't try to change the activation cost by removing your Dragons.

A boast ability can be activated at any point after the creature with that ability has been declared as an attacker. This can be before blockers are declared, after blockers are declared but before combat damage is dealt, during combat after combat damage is dealt, during the postcombat main phase, during the end step, or, in some unusual cases, during the cleanup step.

The cost reduction applies only to generic mana in the activation costs of boast abilities you activate. It can't reduce requirements of specific colors of mana.