A lot of spiders probably already have this. But I found a way to score the achievement a lot easier than I thought I would. Theoretically works on any escort map, but I specifically got this on King’s Row. In the tunnels of the plant with the sidepassages, grapple to the side of the payload so that your feet slip off and quickly click an enemy’ head. I killed a junk rat and scored the achievement .
I’m a dogwater widow but I got it by just jumping backwards down some stairs and scoping (by the exterior mega health pack on Nepal Sanctum). Seems more spammable as the achievement doesn’t actually require that you use grapple, just that you be in the air. So this should theoretically be possible anywhere there are stairs/slope and decent LOS.
I don’t play widow. Looking at scope give me nausea. I read somewhere if a Sigma ult and if a widow without the achievement is in it. They would get it?.
or even easier, you could ask the enemy in quick play to help you. had to fullfill an achievement as support once and i asked the enemy team to help me. they agreed, i did it and the game carried on 
and oh yeah, everyone clapped and i was a hero and blizzard finally acknowledged
and gave me my owl rewards. end
I rarely play Widowmaker and even I got it on my third game.
Grappled the middle pillar and sniped Torb on Eichenwalde only to be followed up by being solo ulted by the enemy Pharah 
I remember getting it through a bit of cheese on Oasis City Center-- the one with the cars. Go to the back with the jump-pad and just bounce up and down til you get it.
If Sigma ults and Widow is caught in it, there’s a moment where you might have a chance to shoot him in the head before crashing down.
Yeah you just need to be on the air.
You can get Concussive Blast by Pharah off a building and accidentally land a headshot, it’s effective.
Its easy to get on rialto on defense by glitching up onto the roof
I am one of the worst Widows but I got this on Kings Row by grappling at the top of a building at the start and while in flight shooting someone’s head on the other side of the fence.
It’s easier than it sounds, you just need to try it from time to time and eventually you get it.
Just keep trying grapple shots it’s good practice anyway, it’s easy to get.