Just like in Well of Radiance. Bubble is made near totally worthless when you can easily get frozen and melee killed instantly. Just make it so that when you have the "Armor of Light" buff (only available when inside of a bubble), you can't be slowed or frozen. You already can't shoot out of it, so at least make it somewhat more of a defensive super.
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No way, it needs to be able to be challenged a d shutdown somehow. Also well of radiance is on solar so it actually make sense and it could be challenged from outside it (unlike bubble where you have a massive chance of dying if you go in there without a plan.