Mode: Any, but Survival is easiest
Map: Any
Class: Pyro
This can easily be obtained in Survival mode. Choose Pyro and use your primary attack (LMB) to kill 15 dinosaurs (any class) without dying. Be sure to stock up on health and ammo between/during waves if you need it. You should get this before Wave 2 ends.
You can also get the Roast Turkey achievement using this method:
NOTE: If you want to get this solo,
DO NOT choose "Solo" from the Main Menu. For whatever reason, achievements can’t be unlocked in games set up through the "Solo" option.
You can, however, set up a game by yourself through the Multiplayer option. From the Main Menu go Multiplayer > Host Game > and then setup your game (make sure “Play Over LAN” is set to “No” as this seems to disable achievements as well).