When his shield is immune it will tell you in the lower left corner (above your super meter) which Psion Flayer is giving him their shield (void, solar or arc). If you miss it when it says it, you can tell which shield he has by the color. You then need to kill the corresponding Flayer to lower the boss's shield. When it goes back up it will pick a different shield. Kill that Flayer. Repeat process for the last Flayer. You won't be able to kill the Flayers until they give their shield to the boss.