Which situations trigger Immortal Phoenix's 'when you kill a unit with a spell' condition?

Ruling: Based on Kelly's confirmation and discussion consensus, situations A (Cull the Weak on your own unit), B (Cull the Weak on opponent's unit), and D (Stormbringer) should trigger Immortal Phoenix. Situation F (Noxian Guillotine's delayed trigger) likely triggers as well since it's still the spell's effect killing the unit.

Sequence:

• A spell kills a unit if the unit dies during the spell's resolution
• A spell kills a unit if the unit dies in the cleanup following the spell's resolution
• Delayed triggers from spells may still count as the spell killing the unit

Nuances:

• C (Stupefy on damaged unit) does not trigger because Stupefy doesn't kill the unit
• E (Last Breath) is unclear - the unit deals the damage, not the spell directly
• There is debate about whether game state actions (cleanup) killing units should count as the spell killing them
• The rules need clarification on when a spell is considered to have killed a unit, particularly regarding cleanup deaths