Fire Shield is very powerful; making it deal 2 damage per round is EXTREMELY overpowered.
Think about it this way. The Rain of Fire card costs 7 unupped, 5 upped. The Shield costs 5 unupped, 4 upped.
If the shield dealt 2 damages per turn, in 3 turns, it is equivalent to playing 2 Rain of Fires.
The quanta used by the Shield = 5 fire quanta. If the shield got exploded, placing another one takes the quanta cost up to 10.
The quanta used by the 2 RoFs = 14 Quanta.
This is very obviously unbalanced. Upgraded, the stats are:
Shield = 4 or 8
Fire Storms = 10
... still a bad balance.
The Fire Shield by itself is still a very strong investment. It completely DESTROYS any fire deck with its weak HP creatures.