It is very difficult for Zimbabwe to chase such a huge run. They are already a very weak team in Test cricket. South Africa batted brilliantly especially Mulder played a devastating innings of 367 runs. He single-handed pulled the team up which can be said to be another record in Test history. South Africa batted by influencing the Zimbabwe bowlers. Their first-order bowlers could not take any advantage from the pitch. At the end of the second day's play, the Zimbabwe team was still behind by 405 runs. Chasing a huge target, they started batting cautiously but opening batsman Dion Myers was out for just 11 runs although they still 9 wickets remain. I think the Zimbabwe team will lose by a huge difference.
You are saying it is difficult for Zimbabwe to chase this score, i would say it is close to impossible for them. Although they had a decent start to the batting in the 2nd innings with Takudzwanashe Kaitano still batting on 34, but they still trail by 405 runs. It is almost impossible for them to first level the score and then take the lead with just 9 wickets in hand.
It is just a matter of how long the Zimbabwe team can survive. Sean Williams had a good innings in the last innings and also some batting were shown by the middle order. It would be interesting to see how Zimbabwe starts the game tomorrow.