If an opponent casts Hidden Blade to kill my unit at a battlefield, can I respond with Fight or Flight to move the unit back to base, and would this prevent the unit from dying?

Ruling: Yes, moving the unit to base with Fight or Flight will prevent it from dying. Target legality is checked again upon resolution, and Hidden Blade does not see a valid target at a battlefield, so the target is considered invalid.

Sequence:

• Hidden Blade is cast targeting a unit at a battlefield
• Fight or Flight is played in response, moving the unit back to base
• Hidden Blade resolves and checks target legality
• The target is no longer at a battlefield, so it is invalid
• The unit does not die and Hidden Blade's draw effect is also cancelled

Nuances:

• The spell does not fizzle (which would imply no resolution); the spell still resolves but with an invalid target
• Hidden Blade's draw effect is also cancelled when the target becomes invalid