The action of this legend must be trigger in the declare atackers step, or can be trigered in any moment during the battlefield phase?

The same question for swarm of bats, and BDK, or other units that says. action: when this unit atacks ……….

Thanks

An "Action: When this unit/legend attacks …" is a triggered action. Triggered actions must be used at the earliest opportunity after the trigger condition has been fulfilled (and can be triggered only once per trigger condition met, so you'll only get to play one tactic for free with Wurrzag per attack). In this case, this is the action window following the declaration of attackers step, before the ambush stept.