Can I Tactical Retreat my temporary unit at beginning of turn to prevent it from dying?

Yes, you can use Tactical Retreat on your unit with the Temporary keyword to save it.

Why this works:

1. The Trigger: The Temporary keyword triggers at the start of your Beginning Phase ("At the start of this permanent's controller's Beginning Phase... kill this"). This trigger is placed on the chain.
2. Reacting: Because this is a triggered ability on the chain, you have priority to play [Reaction] spells in response to it. You can cast Tactical Retreat and target the unit that is about to be killed by its Temporary effect.
3. Resolution (LIFO): Your Tactical Retreat resolves first, creating a replacement effect for that unit: "The next time it would die this turn, heal it, exhaust it, and recall it instead."
4. Replacement: When the original Temporary death trigger then resolves, it attempts to kill your unit. The Tactical Retreat replacement effect intercedes: instead of the unit dying and going to the trash, it is healed, exhausted, and recalled to your base.

Note: This interpretation is based on the interaction between triggered abilities and replacement effects as defined in the Core Rules. While no official FAQ exists for this exact card-keyword interaction, it follows the established rules for using Tactical Retreat to save units from death effects (as seen in RiftJudge FAQ #10096 and #9425).