Three things need to be satisfied to achieve a 1000 hit combo:
- You need to make a lot of hits quickly without any combo-breaking gap between them.
- You need to not get hit while doing so.
- You need to not run out of enemies to hit.
Quickly making hits is most easily achieved by combining Fidget's standard Energy Blast projectile with Dust Storm, in order to produce a lot of projectiles that make a lot of hits, the Dust Storm itself can also rack up some hits quickly if aimed at enemies, but using it simply to enhance projectiles is quite acceptable. The energy used for Fidget's projectiles is replenished by hitting enemies with normal attacks, so mixing in some of those may be required.
Not getting hit is easiest against the basic melee enemy types, so finding a place where there are no other types of enemies is ideal.
In order to not run out of enemies the combined hit points of the group of enemies should be larger than 1000 times your average damage per hit. The first enemies you encounter in Sorrowing Meadow is a long stretch of zombies only, they are late-game enough to have a lot of hit points, but are otherwise very easy to deal with, so they are ideal. The supply of enemies can be stretched by reducing your own attack and fidget stats through unequipping equipment, and if that is not enough an Ugly Pendant can be used to make your attacks very weak. This trick is also useful for keeping enemies alive when "flying" by attacking airborne enemies.
These are just what I conceive to be the ideal conditions for making a 1000 hit combo, lots of different places can be used. Some might swear to a slightly different approach, and that may work fine as well.