Acidfex, huh? I still my Terrorfex (yeah, I know, it's also a DE thing) is better. It has a Barbed Strangler (S8, Ordnence Blast), Sything Talons, and Extended Carapace (2+ save!!! WOOHOO!!!) plus implant attack and Bioplasma. Basically a normal Carnifex, with 2+ save.
The Barbed Strangler is usless on any other 'Nid, so take it on the Fex.
I don't actually have a 'Nid army, I just like making mutantes, so if you need an idea, just ask your friendly neighbourhood C'tan!