Destroy some Spikings, and they'll drop Birsky. The Carryarms that carry the Spikings, instead, not only are hard to catch, but also they are one of the few enemies in the game that don't drop any Cyber-Elf.
Destroy some Pantheon Hunters, and they'll drop Gireff (they are the only enemies in the game that drop a nurse Cyber-Elf).
The Contrainers change the direction of the wind every two hits received. If you spare them, they can boost your speed, but they could also push you into an enemy! They're another one of the few enemies that don't drop any Cyber-Elf.
Destroy some Totem Cannons, and they'll drop Gibber.
Explore the Resistance Base and then re-visit the disposal center. Note that the only enemies left there are those that could drop a Cyber-Elf during the mission.
Cyber-Elves in the Resistance Base
As Cerveau suggests, talk to old Andrew. He will tell you his story, and then he will gift you Clocka.
Leave the base and go west/left. Now the guards at the desert gate left, and Zero can climb on top of the gate. Hit the hidden box and get Lippie.
Cyber-Elves in the Disposal Center
Jump off the tallest building and slash at the box hanging on a hook. When you land, you can either climb up and jump again to get Mippie; alternatively, wait for it to come down and repeatedly destroy the Totem Cannon, just to improve Zero's experience with weapons.
In order to open this box, you should use the Totem Cannon below as a platform, then slash the box from below; forget about the Energy Crystal: it requires a pixel-perfect jump. Instead, wait for Clocpooh to fly down to Zero.