If a card with a 'When this battles' trigger (e.g.: Emergency Defense Field) is played on a character after the attacker is declared, will the "When this battles' ability work?
The trigger for all "when this attacks/defends/battles" is at the same time and happens only once as the attacker is declared and tapped. If you use a "when this battles" trigger on a character to then play a "when this battles" upgrade onto that character, the upgrade won't trigger as it's now too late in the attack for those effects to trigger.

Source: Wizards of the Coast (2020-06-10)    【Link to Ruling】

Tags: Emergency Defense Field, Triggers