This achievement is earned by completing ten waves in a row with a custom (see modded) weapon. You cannot use any other type of weapon, this includes your fists or killing with a kick. Any type of modded weapon should do, but I do believe this achievement is a bit glitched.
I couldn't use all of my weapons that had modifications on them, so this is my tip: Go into the easy arena and use a single weapon for a few rounds. Sometimes, for whatever reason, it may not count wave one for this achievement, but if you do a few waves with just a single weapon, you will eventually be able to check the progress of this to see if your weapon is going to work for this.
If you are at a high enough level this should be no problem by yourself in the easy room. I would suggest having at least one melee weapon and one to two guns if you're on your own for crowd control as infected do start to appear in large numbers. You cannot die during these waves, so medkits are also a must. There is a workbench in the arena so no need to worry about your weapons breaking. If you are finding money to be a problem during this, try to get as high up in the waves as you can on easy with whatever weapons you have, since in later waves the special infected typically drop a few thousand dollars and there are enough of them to raise plenty of cash for you. If you do this in co op, not everyone needs to use a custom weapon, as this is only tracked per person.
If done in the easy arena, there are plenty of good spots, but the best I've found is a ledge next to the bench that is really just a ramp leading up. If you stand near the start of it and slowly back up, you will be able to funnel them without trapping yourself. You can also try using the bridge, but be warned, they can knock you off and since you cannot just kick to guard yourself when your stamina is low as it now costs stamina to kick, you may well die there. Using the one side of the bridge instead of the middle is probably a better idea, since if you stand on the side away from the spawns they will have to cross it and may fall off (this does not ruin this achievement and is quite helpful.)
Any questions, ideas, or issues comment. Thank you and good luck.