You will need the Getaway expansion and will need $15,000 to purchase the melee weapon. Team Deathmatch
is by far the most beneficial gamemode in relation to this achievement, because enemies often spawn directly in your sights. Just chase and mow them down (Professionals stealth training gadget can help if your dealing with very observant enemies).
In particular, Downtown
are the best maps for the achievement. There's a small little shrubland to the side of the cartel mansion in Riptide where I like to run up and down scouting enemies. In Downtown there are a group of elevators; one at C (inside the offices) and another on the rooftop by the building next to it. Just remain out of direct sight as they the doors open and they exit the elevator (normally if they are snipers, they'll try and find a sniping position and you can swoop in for the kill).
Try and stay away from teammates, as they will immediately kill your potential target. Following this, try not to highligh enemies, as again if teammates see and enemy they will shoot them. Dont camp around one elevator, otherwise the enemy team might get suspicious and foil your hiding spot. Try and alternate between these two elevator groupings.
The Fire Axe does have a considerable amount of range to it (not alot), so you dont neccessarily have to be dead-smack in the face of an enemy player. If you see any enemy moving towards you and you're behind cover, enact the melee animation before you move out of cover (because its heavy melee weapon). Be patient; if you see an enemy, dont go charge axe blazing. Analyse the area and ensure you are able to pull of the kill.
Here's a video i've created for reference:
Otherwise, you can just play normally and you will eventually come across a few unsuspecting snipers on your road to lvl 150.