Buff pain damage from acid, a human having flesh literally turned to goo by a bioweapon should not be able to run and shoot. This would also make memey abilities like acid grenade useful as a CC, marines don't currently care about acid grenade damage, primarily due to the fact that it can be healed by a singular dose of kelo, and has minimal side effects outside of a paltry 20something damage.
Or add in a certain amount of compounding permapain, that requires proper medical attention(nervous system restoration) at an autodoc or surgical bed, to restore. Ideally, the meta would shift from pushing over the dead, setting a timer and trying to hold to perma, to crippling marines with a combination of broken bones and perma pain, pushing whenever you break the lines and retreating when they have been treated and are at a battle ready state.