I'm attacking a battlefield, I initially use Facebreaker, my opponent doesn't react, it resolves, I pass focus to them. Can I use Tactical Retreat afterwards to prevent my unit from dying?

Yes, you can use Tactical Retreat to prevent your unit from dying, provided you play it as a [Reaction] before the event that would cause your unit to die occurs.

Timing and Interaction

1. Facebreaker Resolution: You have already played Facebreaker and it has resolved. Your unit and the enemy unit are now stunned (they will not deal combat damage this turn).
2. Focus/Priority: After Facebreaker resolves, the state returns to Open. You must have Focus to play a card during an Open state, or there must be an event on the chain to react to.
3. Playing Tactical Retreat: You can play Tactical Retreat as a [Reaction] provided you have not yet reached the point where damage is assigned or the unit is killed.
4. Effect: Once Tactical Retreat resolves, it creates a replacement effect for your chosen unit. The next time that unit would die this turn, the effect will instead recall it to your base.

Important Clarifications

• Replacement Effect: Tactical Retreat does not immediately recall your unit. It sets up a "shield" that waits for the next time the unit would die. When that death event would occur, the replacement effect intercedes, and the unit is recalled to your base instead of being sent to the trash.
• Stunned Units: Because your unit is stunned by Facebreaker, it will not deal combat damage. However, it is still present at the battlefield. If the combat ends and units from both players are still present, the attacker's units are recalled to their base (Rule 461.1.a.2). If your unit would be killed by other means (such as an opponent's ability) before the combat ends, Tactical Retreat will trigger then.
• Recall vs. Move: As noted in the card text and Rule 450, a recall is not a move. It bypasses restrictions that prevent units from moving.

Summary: You can play Tactical Retreat to protect your unit. It will remain on the battlefield (stunned) until it would be killed, at which point the replacement effect will trigger to recall it to your base.

Note: I could not fully verify this ruling against the retrieved rules.