The Genotoxic Nightmare deals a large Fire based Area of Effect (AoE) damage which will kill runners easily if they are within the attacks range. For a runner to survive this attack it is recommended that they spec reasonable fire resist, and utilise the fire based armours. Any runners who get too close to the Nightmare, are likely to be killed by it's massive Melee attack.
The best tactic to taking down the Genotoxic Nightmare is for runners to keep a good distance, and if the runner is in a team it is important to make it the primary target. It has however some weakness to more advanced weaponry, runners are advised to use Energy and Xray weapons or ammunition.