For the Exotic Catalyst quest, you will need to get Grenade Launcher mode kills with Quicksilver Storm on Neomuna. During Patrol, the enemies are beefier on Neomuna and a pain to kill. To more easily farm the kills I need, I launched the 2nd Lightfall campaign mission from the Neomuna Director page (Under Pressure).
In a single playthrough of the mission, I managed to get all 50 kills :)