I'd say nerf AST's shields to be weaker than SCH's, but let Time Dilation/Celestial Opposition increase their strength. CO would be 50% for the whole party (effectively turning it into Deployment Tactics), TD 50% for one target. In order to stay relatively balanced with SCH, AST's Noct Helios' shield potency would be reduced to 130%, while Noct Benefic would remain unchanged. However, in order to prevent people stacking these cooldowns to create a ridiculous 1.3k potency Noct Benefic shield, these spells would overwrite the old shield with a new "overshield" buff with the increased potency, that could not be buffed further.