The main issue with Dimensional Shield is that many of its counters are rare cards. With all things being equal (i.e. player has access to all/some rare cards) I do not believe that the shield is overpowered. However, for new players, this shield can be quite problematic.
To elaborate, here are the counters that new players have access to.
New Player Counters: Explosion, Butterfly Effect, Momentum, Steal, Psion
Of these, Butterfly Effect, Momentum and Psion can all be countered by a single Lobotomizer.
Other potential counters such as Poison and Firestall are oftentimes too slow, especially if the Mono Aether is running a Wise Dragons build.
Now, let's see the counters available to players who have unlocked more rare cards.
Experienced Player Counters: Explosion, Butterfly Effect, Momentum, Steal, Psion, Shard of Wisdom, Titan, Shard of Freedom, Shard of Focus, Pulverizer
Essentially, more than half of the counters to this particular shield are rare cards that new players will not have access to at the beginning. Shard of Freedom and Shard of Wisdom basically have no counter in the Aether element and, therefore, are usually auto wins. Titan also does not have an Aether counter, but is much slower.
Since half of this cards counters are restricted from them, I can understand why new players can be frustrated by this card.
I feel the best way to balance this card is to add a non-rare Permanent Control spell to another element. Maybe a Rewind Permanent in Time Element. Maybe add another buff to flooding that water creatures in a flooded territory can't be blocked.