[baruk]

The problem as I see it is a combination of the defender advantage and the power of the battlefield wide offensive spells.
I will offer up a new solution: introduce a simultaneous resolution phase solely for the battlefield wide offensive spells.

How it would work:

Defender's turn: Defender moves and casts spells as normal. However, battlefield-wide spells do not take immediate effect.

Attacker's turn: Attacker moves and casts spells. Again, his battlefield wide spells do not take effect just yet.

Battlefield spell resolution phase: Takes place after both sides have moved. All defensive spells resolve first followed by the offensive spells.

eg. In the case of both sides casting master enslave - each unit makes the first MR check against the enemy spell. If the unit fails, it then makes another MR check against the "friendly" master enslave, to see if control of the unit is regained.

Essentially this would make the battlefield spells work a bit like one shot battlefield enchantments, in the sense that they do not necessarily take immediate effect.