In Special Chapter, you should generally hit stage 4 or higher if possible for the best chances of getting biometrics.

When you get 100% of the given stage, you can unlock “no limit” which you can play the specific stage for unlimited times within a 30 minute period.

There is a chance that you will not get any biometrics at all for the level, however that chance seems to be less as you fight higher level stages.

Stage 1~3 will not drop more than 1 biometrics.
Stage 4~5 have chances of dropping 2~3 biometrics.

Guardian of the Galaxy:
1: Modok
2. Yondu
3. Groot
4. Nebula
5. Drax
6. Ronan

Ant-Man Special Chapter
At the time of writing this, the shifter stages have 100% chance at spawning the shifter. However, you do not get biometrics 100% of the time.

1. Modok
2. Ultron
3. Giant Man (Shifter Giant Man)
4. Yellow Jacket (Shifter Ant Man, Wasp)
5. Modok and Ultron (Shifter Ant Man, Vision, Wasp)