I am not the greatest Halo player and have tried 7 times to get through the "Keyes" level and kill all the Elites but the Achievment doesn't pop.
How many of the tall mean looking dudes are there?
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A lot. You should flip the switch in your mind to literally go hunting. Search EVERYWHERE. Right before you take the lift back up to the ship, those elites in the circle like to hide in the lake of coolant if they jump off. I found 2 hiding there. They won't even attack you, so you have to find them.