I like the antag gameplay, but I like your suggestion as well. The antag shouldn't be able to finish the job. We had an Alicia that nailed the old guy against a wall and just jumped and dropped bombs.
Perhaps the higher the difficulty, the lower the antag can bring the old man. For example on 50 difficulty, the antag can destroy the shield and 50% of the old mans hp, everything after that, needs to come from grunts.