Talk:Ignis/@comment-191.249.28.156-20160306175950/@comment-172.15.16.99-20160308012246

Blast, yes. Radiation, no.

Ancients, as you say, use Fossilized health type. That type is -75% resistant to Radiation, so your Gas damage is almost doubly as effective as Radiation against them. (I believe you're thinking of Alloy armor, which is +75% weak to Radiation, but yeah, that's not what Ancients use.)

Blast is a good idea, though. It will allow you to deal extra damage to the Ancients, and the proc from Blast, while not as effective against Inested as it would be against biped enemies (i.e. it knocks down Grineer/Corpus about twice as long as Chargers and such) is still pretty good CC.

Alternatively, keep Gas on your Ignis and bring a frame that does the kind of secondary damage you want. For Ancients, you want either Blast or Corrosive (but don't put Corrosive on your Ignis because the multipliers are neutral against everything but Ancients and the proc is useless against Infested because they have no armor anyway.)  I am not sure exacty what all frame abilities do Blast damage, but I know there's a few. I believe Sound Quake is blast damage (and is also decent CC for use in Infested Defense, in a pinch.)  There are others too. I think Rhino's Stomp is Blast damage, so maybe mod your Rhino for maximum Power Strength and just use Stomp for the Ancients and stick with Gas on your Ignis.

And there's always Miasma. Fight poison with poison, right?