The fastest way I've found to get this achievement is by exploiting the game's checkpoint system in the "Death, Baby" level. I did this on both the normal and easy difficulties, so I recommend just playing on easy to simplify things.
Before trying this, I HIGHLY recommend having purchased the Pulse Rifles, which will make this go significantly faster.
About 1/3rd of the way through the level, you'll be asked to choose one of three statues. Pick the first statue, which is green in color. I believe this is the Hear No Evil statue.
You'll go through a portal and fight two waves of enemies. After beating them, you'll have to avoid a series of grenades and explosive barrels for 20 seconds. Once the timer is up, an elevator will drop down.
Ride the elevator up and deal with the next round of enemies on the second floor. Once you've done so, a second elevator will appear.
Ride this elevator up. The cavern curves ahead of you to the right and a long series of the exploding clones will spawn at the end of the hallway and begin running towards you. Take them out as fast as you can, then collect all the DP tokens they've dropped.
This rewards you with a huge DP bonus for the kills AND a large number of tokens dropped by the enemies.
After you've picked everything up, go into your upgrades menu and buy an upgrade. You can buy more than one if you like, but I don't recommend it until you've banked up a very large amount of DP.
Once you do this, press the start button and pick "Restart from Last Checkpoint".
Make ABSOLUTELY SURE you repeat this process each time!
The game will start you back right before the wave of exploding clones has spawned, allowing you to farm exponentially increasing amounts of DP to upgrade with. The game creates a checkpoint automatically each time you purchase an upgrade, allowing you to repeat this process until you've maxed out Deadpool's upgrades.
I recommend buying the Greedy, Greedier, Power and More Power upgrades for the pulse rifle first to make this go even faster.
Hope this helps you guys out!