Use the scanner when the arena goes dark
During the Possessed Amari fight in the Conduits, activate the scanner to reveal Amari’s outline in the darkness. Use that outline to find the boss instead of firing blindly into the dark arena.
Check Activate the Scanner in your Controls menu for your current binding. Keep the scanner and target lock distinct: scanning reveals the outline; target lock helps you keep an enemy selected while moving.
This mechanic applies to the darkness section of the later Possessed Amari fight, rather than the scripted encounter at the beginning of the game.