Yeah I got it from the first 2 wraiths, in fact I was playing on normal, single player, and I was like LEROY JENKINS and armor locked till they both were right next to each other and dropped the hurt on them, bam first try...
The only achievements I have a hard time with are the pistol achievement which I couldn't get till I restarted the campaign for some reason, I have it now, and the other is If They Came to Hear Me Beg... I start on Pillar of Autumn and jump on the elite down the mountain and my feet still registered as hitting the ground, so it doesn't matter how many times I do it... also never jacked a Banshee