Now i did this solo and have found a few ways to grind money from waves i was getting up to around 30,000 cash before even wave 8. And had finished at around wave 13 with 52,000 points.
Solo Method - First choose the map resistance it is the first map on the list. You will start off with a pistol during this entire match the only thing you may need to spend money on is ammo this will be due to a juggernaut and two helicopters that eat up your supply. Now at the start look at the bottom left hand corner of the screen and you should see two bars with names on top. These are bonuses for doing the specified requirments and are a big boost to the achievement. You will want to only focus on these couple ones.
Killing spree - Kill multiple enemys in a row without getting hit. if you get hit the bar resets.
Rampage - Kill multiple enemys in a row quickly if you stop killing the bar will start going down.
Headshots - Get multiple headshots during the round.
Knife spree - knife as many enemys as you can if you stop killing the bar starts going down.
First complete bar - 500$
Second complete bar - 1000$
Third complete bar - 1500$
Was never able to get over three bars due to there not being enough enemys.
The easiest way to do these and get the biggest boost is to first wait untill the red dots indicating the enemys show up on the mini map. Then lead them all together by running towards one group of enemys and staying close to them. (they will not do much damage the first 8 waves or so after that dont get to close to them but lead them into a group) Once most of or all of the enemys are together and you know what bonuses you can aquire focus on only killing in those methods. This will give you anywhere from 500 to 3000 cash boost not to mention 100 for every kill. You will want to pick up every wepon the enemy drops it will start out with a Model 1887, then Mp5, then ak47, then Acr 6.8 and so on.
When it hits around wave 6 or so two mini birds will fly in you will want to take cover in a building or under one of the arches. Using the Mp5 shoot each bird making sure to go in and out of cover when they start firing back. If you run out of ammo run to the armory marked as a gun symbol (will be a pistol) on your screen and choose the first option refill for 750.
Continue using the same method as above untill wave 9 or 10 when a juggernaut will be dropped in. Make sure to keep picking up ammo and if you see the enemys are using a different gun switch one of yours for theirs. The juggernaut is easy to kill just lead him into a stright away where you have cover make sure its not a car that can explode almost died on wave 20 with 3 of them due to that. Use the cover to shoot at the juggernaut with all of your ammo and if he gets close run further back to another cover. This allows you to hit him but he will not be able to hit you. Once he is dead run to the armory and refill your ammo again.
There wont be any boss enemys for a while now so you should be able to continue the point method and achieve 50,000 cash by wave 13-18. If you cant continue till wave 19 or 20 and 3 juggernauts will be dropped in this is a little harder but use the same method as the first juggernaust just make sure to lead all three in the same direction so as not to have one behind you and infront of you.
Wanted to add once you get the achievement you can spend all the 50,000 on as much weponry as you want. I had used it to buy everything in the epuipment armory and most of the wepons in the weponry armory for the other achievements. If you need help with those i will have guides posted for each one also.
Co-op - Thought id just add what everyone seems to be doing in co-op to achieve 50,000. I have yet to do this in co-op but ill give a few helpfull rules. Now in co-op you can use many of the same methods as i did in solo but it may be more difficult since there are more enemys per a wave then solo.
The main way people are doing this is once both players have a total combined amount over 50,000 one player transfers there money to the other for the achievement to pop. This may be difficult due to trust issues if doing it with random people. I suggest if you cant do it solo try to get a friend to help or set up a match on trueachievements to find another like minded player with there easy to do match setups.
To do this once you have the combined total of 50,000 (can use a calculater if you need to to make 100% sure.) both players make there way to one of the three armorys located on the map. The wepon armory marked as a pistol on your screen, the eqipment armory marked as a grenade on your screen, or the air/perk armory marked as a plane on your screen. Once at one of the three stations one player open up the armory and look at the bottom of the window that pops up. It should say share 500$ with teamate. Click X to share and it should transfer 500$ of one players money to the other. Continue doing this untill on play has reached 50,000. When that is done it can be retransferd to the player that has yet to get the achievement in the same method.
Hope this helps thumbs up if it does thank you! If anythings wrong comment and ill try to fix it.