Resident Evil: Operation Raccoon City

This achievement is gained by killing 103 enemies with incendiary rounds.

Incendiary rounds are rounds which when fired will set enemies on fire upon impact. This means things like barrels and incendiary grenades will not count towards the achievement.

To be able to use incendiary rounds you will need to choose Lupo at the character select screen and purchase (If you haven't already) 'Incendiary Rounds Level 1' in the Active Abilities section. This can be upgraded to level 3 which will allow you to fire these rounds for 15 seconds. After those 15 seconds you will have to wait 30 seconds for the ability to restore.

So if you have played through the game and not kept this achievement in mind I would recommend playing on casual difficulty on the level 'End of the Line' which is the last level in the game. Also when going for this I used the 'Machine Gun' mainly because of it's high damage and high ammo.

When playing 'End of the Line' after killing 5 or 6 of the infected you will come across an area where you need to climb some steps and walk across some red and blue containers. When at the top of the steps you will notice 20-25 infected enemies wandering around. At this point it would be a good idea to take out the spec-ops sniping you from across the area and then use the Y button to activate incendiary rounds. Bare in mind that these enemies will need to die from your incendiary round and not from your squads ammo.
If you are using level 3 incendiary rounds you will only be able to fire for 15 seconds before having to wait for the ability to restore so I would recommend spraying a group of zombies with the rounds for the 15 seconds then retreating to the bottom of the steps you climbed not long ago.
Simply rinse and repeat until all zombies have been killed.
Proceed onto the next section using the ammo box near some steps and then you will find 15-20 more infected enemies and more spec ops snipers so do the same again.

At this point I would recommend exiting the game and restarting the level. As long as you don't quit to dashboard or turn off the console all of your kills will be saved.

Hope this helps and if you feel I've missed something or made an error let me know in the comments. Thanks.

Level 6 - Vertran - Shotgun

Lupo - quick reload level 3
Flame rounds of course level 3

Kill your team mates as soon as you spawn and when you get down stairs on the platform to activate the elevator there will be 20-30 zombies and some more after you use the lift, press y when up to these zombies and be close and just shoot the zombies and reset the game over and over again and I got it after 3rd try :)