Combos are performed by attacking enemies and chaining hits in quick succession. After hitting an enemy, you have a few seconds to score another hit before the combo ends. Taking any damage will also cause your combo to end. Your combo counter is at the top right of your screen.
This mostly involves repeatedly firing projectiles in combination with dust storm on large groups of monsters. Projectiles track enemies and hit many times to increase your combo quickly. If you're already very high level and find yourself doing too much damage, unequip any items that add damage. You can also find an Ugly Pendant and equip it to lower your stats to 10%. Try to end your combo shortly after your combo counter reaches 1000 hits. Getting hit and having your chain broken even after surpassing 1000 hits will NOT earn you the achievement.
This can be done very easily, very early in the game on the first two giants you fight. If you're already past that part, you can also get it fairly easily on Rock Hounds in chapter 2. You'll regularly run into big groups of them. Further still, in chapter 3 you'll constantly run into big groups of zombies which should also make a thousand hits easy. My combo of choice is +
As everybody above says Sorrowing Meadow is the best. I'll give a little more detailed directions as to how I did it at level 35 with balanced stats. If you enter from the right side near the save point and 2 exploding zombies you will come to a pit with vines on both sides. Before you go down do yourelf a favor and take some damage before you go down. I had 2200 HP so I dropped it down by half, but make sure you use the best armor WITHOUT regen on it. You'll also want some low end healing items. The reason is everytime you use ANY healing items Fidget's magic is refilled. On Tough difficulty her magic runs out fast. Once you go down into the pit take out the flying insects and the hive, but avoid everything else. Bat everything else over to the edge and have them drop down. You can do the
Aerial Dust Spin over all the zombies and the big bugs over and over. Once your magic gets low or you start to turn red land and pop a healing item. Jump and repeat. I was able to get a 4200+ combo several times. I had everything unequiped and had the stat reducing amulet on. You can probably do it at higher levels and without using the Ugly Amulet and you;ll still get the 1000+ required.
Equip the Ugly Pendant to reduce your damage and go find a huge number of Zombies usually on the first screen or second of the Meadow, and just keep spinning your weapon over and over without damaging yourself by consuming an item or using your standard attack for a few seconds.
All you gotta do is use lightning and search for those things that spawn unlimited zombies in Sorrowing Meadow. Just keep tapping The Y and RB button for a crazy multiplier. Make sure to have no Zombies behind you, as those are the ones that usually break you combo. Remember to keep moving around too. One spot will not do you justice. I'd say: attack, fly, move around, attack. Also, keep away from those exploding zombies. They will seriously hurt you.
I found it rather easy to just aerial dust storm while fighting two giants on Tough. Keep the giants together, zip back and forth a few times, and just be mindful to pause here and there before getting too red and losing your combo.