Flamethrower

Enemy units taking hits from weapons with this special rule get -X to Defense rolls.

Models with this special rule must accumulate X wounds before being removed as a casualty.

If a model with Tough joins a unit without it then you must remove regular models as casualties before starting to accumulate wounds on the model with Tough.

When a unit with multiple Tough models takes wounds you must accumulate them on a single model until it is killed before starting to accumulate them on another.