Can Last Breath be used to ready a unit when the opponent doesn't have a unit to target for damage?

Ruling: No, Last Breath cannot be activated without both a friendly unit to ready and an enemy unit at a battlefield to deal damage to. Both targets must be legal at activation for the spell to enter the chain.

Nuances:

• If Last Breath is successfully activated with both valid targets, but one target becomes invalid before resolution, the spell resolves doing as much as it can with the remaining valid target ("do as much as you can" only applies during resolution, not activation)
• The distinction between activation requirements and resolution is key: all targets must be legal when activating the spell, but targets can become invalid during resolution
• This differs from cards like Dragon's Rage which use "Then" to indicate sequential timing, where the second target is chosen during resolution rather than activation