SC2Mapster

You put the set under the Search - Areas - Effect field of the Hellion - Infernal Flamethrower (Search). If you put both parts used by the set effect under the search it will count them as different searches since that effect can check several areas with different filters.

For your second question use markers. Just like the Damage effect of the hellion flamethrower there is a validator called No Markers., add this to your Apply Behavior effect. Markers basicly count if a unit has already been targeted by the same effect in an effect tree previously per cast.

Did you replace the damage effect with the set effect in both instances? Try setting the buff duration to 0 (infinite) and giving it to the marine weapon as a direct effect, see if the buff is applied then.

It might be easier to make a new buff that slows units, it is pretty simple to make.

You absolutely sure you removed the validator from the Apply Behavior effect?

The Action type actor for the attack requires the Damage effect of the weapon to be created (required for visuals). You find the Create Persistent effect has both an initial effect and offset periodic effects (effects at a distance). The periodic effects use a Search Area effect that then uses the Damage effect.

The marauder applies the slow buff using an Apply Behavior type effect called Marauder - Punisher Grenades (Apply Slow Behavior). You want to create a new Set effect containing the Marauder - Punisher Grenades (Apply Slow Behavior) and Hellion - Infernal Flame Thrower (Damage) effects in the Effect - Effects field. Under the Create Persistent and Search Area effects of the hellion weaapon you must swap the Damage effect with the Set effect you created.