Personally mine is [i]Overcharge[/i] . Kill two enemies at once with the fusion rifle , 20 times. It might be easy but it's still irritating as hell.
Edit: excluding exotic bounties. I know some of those are considered extremely tough.
Edit: everyone's suggesting the first moon mission seems like a good method for the overcharge issue. Thanks guys. That's my problem fixed anyway!
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Post Mortem and 20 double kills with the fusion rifle.