To gain this achievement YOU must collect $1,000,000. This means that it is not team oriented per match. You must actually pick up the cash off the ground to be counted towards this. This means you will be playing a lot of MVM as a Scout. TYhe Sniper is capable of gaining money as well but it only counts if the red money is left to disappear on its own. If someone picks it up, it counts towards their achievement.
This achievement doesnt take too long at least it in comparison to the Bot killing achievements. To start playing Ghost Town is probably your best bet. There is a total of $8000+ on that map. Playing as a Scout I managed to get between $5,000 - $7,000 a match if you complete the wave. Obviously your skill and teams skill will impact how often you complete the wave.
We will start with the loadout:
Primary: Soda Popper or Force of Nature
Secondary: Mad Milk
Melee: Fan O'War
Soda Popper and Force of Nature both have a -66% clip penalty which isnt ideal however the advantages of these guns outweighs the negative. Both the Soda Popper an Force of Nature have high fire rates and reload times which removes the upgrades from your list allowing the money you would spend on those upgrades to go elsewhere. Both guns also reload the entire clip in just one animation which is fantastic. The Force of Nature has a -10% Damage Penalty but also contains knockback on player and enemies which can be useful. Soda Popper has hype which allows multiple mid air jumps
Mad milk is pretty much your only good option. The pistols are garbage and should not be used. Bonk can be useful for grabbing cash but it hampers your speed when it ends and you can just use an ubercharge canteen instead. The Mad Milk will allow you and your team to recieve health back based on the damage you or they inflict, it can be upgraded to slow the enemies down by 20% and can extinguish fires.
Fan O'War is great because it marks the enemy for death making all hits mini Crits. The sand man does the same thing but requires $500 to do the same thing the Fan does for free. None of the other weapons matter.
Next Upgrades. I recommend:
3x Explosive Resistance Upgrade
3x Bullet Resistance Upgrade
2x Mobility Upgrade
3x Fire Resistance Upgrade
3x Critical Resistance Upgrade
2x Clip Size upgrade
Bullet Penetration
2x Ammo Capacity Upgrade
Mad Milk Speed Decrease Upgrade
Your Primary job as a scout is to stay alive and get the money. Providing Damage is your secondary job. The first set of Bots are Demoknights, Pyros and Scouts with uber bats. This is why the Crit and fire resistance is important. Ue the Mad Milk to slow the demoknights. The next set includes Rapid fire Giant Soldiers with Medics. At this point you arent going to see pyros until near the end so that upgrade can be removed and the cash put towards increasing your clip size. It is important to take the medics out first. The third set will be Soldiers, Heavyweapons, Giant Rapid Fire Soldier and Giant Heavy Weapons. It is important here to have both Explosive and Bullet resistances maxed. Once Clip size is full go ahead and upgrade Damage
After that you will have your first group of Tanks and scouts. Focus on the Scouts and collect the cash they drop and keep an eye on the tanks health. When your team gets the health low run towards it and make sure you get the cash when it explodes. Get used to this strategy. You will be using it for all tanks except the final one as it does not drop cash. Next set is giant demos with demoknights and 1 Tank. Make sure you get this tank as it drops $600 compared to the $200 or $275 the others drop. The set after that will be Buff Banner Soldiers followed by Giant Uber Heavy Weapons with a Giant Medic
Next set is probably the hardest. This is the Spy Wave. You will get 4 tanks and 96 Spies on this wave. Run around collecting the cash from spies and grab the tank money when it blows up helping when you can. Pick up 3 Crit boosts to help when the tank is getting close. This is where you will mostly have your runs ended.
After that wave you will face your last 6 waves that comprise of Uber Bowmen, Uber Soldiers, Uber Demos, Uber Pyros, Uber Scouts with a Tank(No money from this tank) and Uber Heavyweapons with Uber Medics. The strategy for these waves are all the same. Stock up on ubers for 8 seconds of invincibility, use it to collect the cash, try to survive as long as possible die and then rinse and repeat. It does not take much to die here so you must grab the canteen boosts.
All in all it will take you about 166 waves at 30 mins a wave if you average $6,000 per match.