So I had about 30 minutes to get this challenge before leaving for class. Bonus moshpit wasn't helping me so I went to ground war and we got Raid domination.
I tried to get the across the map tomahawk kill form the A spawn, but I couldn't get it. I think it was tough because the C spawn has that choke point with a lot of roof overhang, so I had trouble threading the needle.
I was just about to quit, then decided to stick around for the start of the next round. This time I was at C trying to pick a guy off at A. A doesn't have any obstructions so I figured it'd be easier.
Sure enough, I threw those hatchets and within two seconds I see: Chopped up, first blood, long shot, Hail Mary challenge complete, All Humiliation Unlocks complete.
I almost fell out of my chair.
tl;dr: For Hail Mary challenge, go to ground war and get Raid domination and spawn at C, and then throw at A at the beginning of the round. There will be about 8 people waiting for that hatchet to come whizzing into their spawn.
So I just watched a tutorial on Raid and the guy had some good search and destroy and domination spots. The only problem though is that the domination spots work, but you can't get to them right at the first spawn of the round.
By far the fastest way was just to throw TWO hatchets at A from the C spawn. There are no obstructions in the way, and ground war will give you 8 people to hit...........and with the two hatchets thrown you actually have a good chance!