During attacks, abilities with the same timing are resolved in the order of core/mission rules, attacker, defender.
The link contains the relevant rules and all attack-related abilities to date. Also see the attack reference I built from the core rules material. https://www.boardgamegeek.com/image/3458198
Power to Shields: (last chance just before step 7 for Pierce, and anytime during step 7 for other of the listed keywords)
