So this trophies a pain in the ass.
You need to get the 500 kills with your Grey Warden character, if your companions kill a darkspawn then it doesn't count towards the trophy. If you play the game normally and try and finish every side quest you'll probably end the game around 300-400 kills like I did.
MAGE SOLUTION
*** Spoiler - click to reveal *** During the final battle you will have the option to go save the alienage. Go there and save as soon as the cutscene is over. There will be a hoard of 50+ darkspawn you have to kill and they almost all die in 1 hit with the exception of the Ogre and a few other variants. If you're a mage you can literally just go up to the big wooden gate and spam all your AoE attacks and kill dozens of darkspawn very fast. Once you kill all of them reload the save you just made and repeat.
For the other classes you use the same advice but obviously it will take a bit longer.