First, go to Recreational Yard and kill 10 to 20 zombies using your tomahawk. But, you had to do this in a specific location in order to get a blue tomahawk kills. Check the video below for the specific location and this are all on that location.
Second, Hidden head dog. This has a 9 locations and can be seen by your spirit shield. Once you found yours, thrown your tomahawk and the head dog will disappear.
Third, Spirit Dog. To get this, you need to be on the dog round. Then, use you Zombie Shield to see where he is. The bad news is you need to look all over the map. Once you see him use your spirit blast to kill this dog. Don’t forget that you need to charge your shield in order to get a spirit blast.
Last, go back to the underground passages where you get your regular tomahawk and you will see your new blue hells redeemer.