What does the zombie dragon's -100 fire resistance mean?
Resistance means they take less damage, NEGATIVE Resistance means they take MORE damage.
... I cannot move my character fast enough to avoid the dragon and the breath.
With practice you can anticipate the poison cloud and move away from the dragon before it hits. Companions don't have the AI to dodge clouds so that can be a pain.
Is there any way to get better toxic resistance?
The Wiki can point you in the right direction for where to get any dear type: https://www.exiledkingdoms.com/wiki/ind ... itle=Toxic
It's not too hard to get Toxic Resistance into the 130 to 160 range. Add the +40 toxic resist from a Greater Toxic Resist Toxic, and at or near 200 is very doable. Note: 210 resistance is complete immunity.