First, these are demon scouts. Typically, scouts are lighter, faster units than the other beings in the army that they are a part of. If everyone has trouble against scouts, then how the hell can the army be turned without a Macguffin?
Second, in terms of the antidote idea, it's basic practice. You keep using a poison on your opponents and they survive, they'll start to grow antibodies in resistance to it. You change the make up after every fight, you prevent that from happening, letting you keep having an impact with it. Antidotes for the previous toxin will work, but unless you whip something up in the middle of the battle, itll make impact.
Third, I gave him three kinds to keep him from being outmatched easily by a vareity swarm of heroes that would probably seek out the ten foot giant first. But, if you need it, look at my post again. I mentioned his mask. He needs to constantly inhale his toxic fumes in order to survive and breath. Hence the suit to channel the toxins into the carrying tube on his back.
Fourth, I would've made a SU for him, to get his power checked out first, but I had no time for that, and you, Bron, wanted me to post him as an NPC when I mentioned him. Now, I don't have the time to edit the post. I have less than 3 hours before my flight, and I rather be spending time with something more meaningful than your dying RP
Also, I mentioned it before in mid-June, but I deleted the post to not double post and post that I'll be away till the 21st of June, leaving today. This is a bad idea, it'll just eat more time away from the main story and be filler. My opinion.