This will probably be mine as well. Since Killzone shadow fall takes 150 hours+ lol. Either way going for it too :)

 Play Decima (third level) on Veteran Difficulty using the ship Nemesis. Be disciplined with how you kill your enemies, ie don't kill an entire wave at once; give them time to respawn while you're killing so you constantly have enemies on screen. Only use your bomb in a very desperate situation, and make sure to use boost as much as you can. When you reach the boss (big red cube thing), focus on hitting the gray cubes he shoots out. If you can get through the entire level like this without dying at all, you should reach 15 by the middle of the boss fight. I got it on my first try doing this. 

I didn't think that was in Mercenary, I thought that was just in 3. I remember that... I was playing the multiplayer, then I decided I would wait a few weeks and give this trophy a go, then I never played again

That was in Killzone 2...

I don't understand that because Resogun is free.

Exactly. It'll be with every PS4 but not everyone will go for/get the trophy.

Cool. Resogun is such a fun game! I 100% Contrast a couple days ago. I am going for Resogun also I am at 70% or so.

I thought your multiplier carried over to the next level as long as you are playing on Arcade mode?  Also, sidenote, I heard the 1CC trophy is gettable in CO-OP.  but I haven't tried for either of these yet so I could be wrong.

Hey can someone get me that platinum trophy, I give you a few bucks for it