The dragon is vulnerable to blind. With enough int (17 I think, easily reachable), the blind has a good success rate (near 100% if you combine with other spell such as enfeeble or divine light). All it takes is 4 Ap every 3 turn to remove the void dragon from the fight while you take care of the summon.
The main difficulty comes from the storm of status effect the void dragon inflict on its first turn. It helps if you have some defence against these type of attacks with enough bodybuilding/willpower as well as a level 6 leadership character. Counter measure such as cleansing water, purifying fire may help as well. Some orange item also grant immunity to disabling effect such as stunning.
