Easy mode is a good place to start if you want to get into speedrunning this game. I have optimised easy mode and below are videos for all the boss fights no damage and under a minute.

Speedrun for easy mode.

Australis First Encounter:

In first person mode destroy the two crates in the centre of the room and then equip tempest and use a limit breaker. Repeatedly use a laser charge shot whilst spamming tempest.

Rigel Domain:

Use a limit breaker and then immediately on entering the room fire tempest to kill the Rigel. Then jet to the boss and walk around it whilst firing laser before more Rigel spawn.

Regulus First Encounter:

You don't even need to touch this boss. Just go to the other side of the room and wait for him to catch up to you and the cutscene will trigger.

Regulus Second Encounter:

Similar to the first encounter with Australis. Just use limit breaker and spam tempest and Laser charge shots. Try to avoid shooting his back armour as it won't have any effect.