Edit: Seems like a few people are voting negative because they're not getting the challenge to unlock after passing a 1000 combo. To clarify, the challenge won't pop until your combo ends because the game doesn't recognize the combo or add it to the leader board until after it is finished. So after have 999x on the screen for 10 seconds or so, stop doing anything that will contribute to a combo (kills, bounce, airdash, etc.) and the challenge will pop.
Go to your log and start the mission "Getting the band back together."
Finish the first part that has you towing the boat. The next part is the drum section that has infinite OD that spawn at an alarming rate (the same place you go to grind your weapons to max level really easily).
Simply equip your favorite gun with the amp "Second Amendment" so you never run out of ammo and start bouncing on anything while blasting the OD as they swarm you. The best guns for this are the Hairspray Bomb, Acid Sprinkler, ProPain Launcher, Proximity Mine, pretty much anything with AoE will get your combo up at an incredibly high rate.
You should get this in a few minutes. Easy!