Why, Tanya, the force field lasts for two turns, which means you can use any spell in between ;)

Yeah. 2 turns. On the first turn, you cast that "force field." On the second turn, you cast, for example, the suggested "disruptive ray." And on the third turn, those azure dragons fly up to you (as in the example) and you're dead! =) You didn't have time to cast the "force field" again. So, I'm saying that either you need to move faster than your opponent, or you need to cast the force field every turn...
This tactic was described in detail by Sage (I think) in the Best of the best tournament. And it was explained why speed is important.