Gunfire Reborn

The Miasma Effect is caused by combining Shock and Corrosive status damage. The icon for it will be a little toxic cloud over their health bar. The easiest way I came across getting this was by having a shock weapon and combining it with Cat's smoke grenade, as that deals corrosive damage as long as enemies are in the smoke.
The process is to freeze an enemy in place with a shock orb, and then spamming grenades for the toxin. You can also just shoot an enemy with a shock weapon to cause shock , and then spam Cat's grenades at groups of enemies. To make it even easier, grab the upgrade that gives your Orb the same status as your weapon, if you get the chance.