Combo 1000 Achievement
Get the 1000 combo!Note that, while the base game achievements require combos of 500 and then 1000, the first title update has achievements for 250 and 666, and the third title update has achievements from 1500 all the way up to 3500. As a result, use the strategy below but don't stop when you reach a combo of 1000 for this achievement. Continue on until you reach a combo of 3500.
By far, the easiest method of getting extremely high combos is to play Endless Hell mode. There is a nonstop massive swarm of enemies coming in from the top of the screen, meaning it's virtually impossible to lose your combo as long as you keep shooting. On top of that, there will be powerups galore, meaning you'll be shooting in multiple directions hitting enemies everywhere, allowing your combo to shoot up at an incredible rate. Because you'll be getting health powerups too, you'll be able to survive plenty long to get a high enough combo for all related achievements. No real strategy is necessary; just hang out near the bottom of the screen, strafing back and forth, and focus on picking up powerups rather than worry about where enemies or bosses are. They will get shot unavoidably.
The following video demonstrates this strategy and how fast and easy it is to rack up a huge combo without much effort. Full credit to its creator.