Here's my video:
This is a quick guide to show where to throw axe from and where to aim.
Easy to do solo but it can be glitchy. I got 5 kills multiple times, in same round and additional rounds before it unlocked. I don't know why it glitched but I got the trophy the round immediately after a warden round (where 2 spawned). So my thought is if you are not getting the trophy, keep going until after a warden round.
Also, when collecting souls to build Hell's Redeemer, i found that throwing the axe without charging it beforehand worked better than charging it.
Don't know how to build the Hell's Redeemer? A great quick guide for that can be found here:
Enjoy