This is one of the HUGE misconceptions people have about zileans bombs...
The first thing everyone does when they have a bomb on their head is run away from their creep waves and hide by the turret. This is wrong, instead run on top of your creeps so the bomb kills or at least removes much of their health.
Because now you are 'pulling' instead of 'pushing'... You are forcing zilean to sit just outside his creep wave for exp near your turret, overextending himself. This is the perfect time to setup a gank with your lane partner and/or jungle.
Just don't give him unnecessary creep kills and be careful that you aren't putting yourself in a position to get killed by his laning partner.