Anti-Mutation Station Trophy

The Pathogen creatures first appear very briefly during the mission "Gift of Fire: Advance" but are easy to kill, especially if you have the Incinerator Turret upgrade unlocked for the Technician kit. You will easily rack up 300 kills within 2 playthroughs of Campaign 3 "Gift of Fire" so I wouldn't worry about this trophy. There's also a Daily Tactical Opportunity that sometimes appears, which has you kill 400 Pathogen so it can be used to kill 2 birds with 1 stone if you get lucky to have it appear before getting this trophy. Note: There is a Daily Tactical Opportunity that has you kill 400 Pathogen so if you're lucky enough to have that appear before getting this trophy, you can use it to track your kills.