OK, so the essence of this cheat is pretty much the same as the Infinite DP (easy) cheat (see list below), except we do a little bit more stuff to maximize your DP earning and make it as efficient as possible.
Do the tutorial and kill everyone and then make sure "Greedy Perks" is the first thing (upgrade) you purchase. This will give you more DP from kills.
Following that, when you go down the sewer pipe, kill the enemy indicated by stealth and then go around collecting all the DP tokens you can see.
Then it's time to buy a really cheap upgrade, which will force the game to save. Right after that happens (circle in the bottom left) pause the game and "Restart from the Last Checkpoint". This will let you collect all the DP tokens once more!
When you're done with that, buy an upgrade for 50K DP or under and then do the same thing - save, pause and "Restart from the Last Checkpoint". Keep doing this again and again until the cheapest upgrades that you can get cost at least 75K DP.
When that happens, pause the game and do the "Restart Chapter" option. Then build up your DP once more by simply killing enemies. Then when you get enough DP again, go back to the sewer/stealth/token area and grab all the tokens.
Then simply keep doing this over and over until you get everything - all weapons and upgrades!
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