Dragonkin Berserker — ruling 1

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.

Dragonkin Berserker — ruling 2

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.

Dragonkin Berserker — ruling 3

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.

Dragonkin Berserker — ruling 4

If it's not your turn and you gain control of a creature with a boast ability after that creature attacked, you can activate that creature's boast ability if it hasn't been activated yet that turn.

Dragonkin Berserker — ruling 5

If a creature with a boast ability is put onto the battlefield attacking, it was never declared as an attacker. Its boast ability can't be activated that turn.

Dragonkin Berserker — ruling 6

If an effect adds additional combat phases to a turn and a creature with a boast ability attacks more than once during that turn, its boast ability can still be activated only once.

Dragonkin Berserker : règles, fonctionnement et interactions (MTG)

Dragonkin Berserker is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

First strike Boast abilities you activate cost less to activate for each Dragon you control. Boast — : Create a 5/5 red Dragon creature token with flying. (Activate only if this creature attacked this turn and only once each turn.)

How it works : Dragonkin Berserker

Original English text — Wizards of the Coast / Scryfall

#1February 5, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#2February 5, 2021· Wizards of the Coast
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.
Card named in this ruling
Dragon
#3February 5, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#4February 5, 2021· Wizards of the Coast
If it's not your turn and you gain control of a creature with a boast ability after that creature attacked, you can activate that creature's boast ability if it hasn't been activated yet that turn.
No other card is named in this ruling.
#5February 5, 2021· Wizards of the Coast
If a creature with a boast ability is put onto the battlefield attacking, it was never declared as an attacker. Its boast ability can't be activated that turn.
No other card is named in this ruling.
#6February 5, 2021· Wizards of the Coast
If an effect adds additional combat phases to a turn and a creature with a boast ability attacks more than once during that turn, its boast ability can still be activated only once.
No other card is named in this ruling.