Can you play around retreat by using Hextech Ray on a 4 Might unit and then using Stupefy to kill it?

Ruling: You need two sources of might reduction (or effects that would cause lethal damage), with at least one at reaction speed, to play around retreat. If you deal 3 damage to a 4 Might unit with Hextech Ray, then play Stupefy, the opponent can retreat in reaction. You would need a second Stupefy to react to their retreat, which would kill the unit when it resolves (assuming no other responses).

Sequence:

• Deal damage with Hextech Ray (or similar spell) to a unit
• Play first Stupefy (or other might reduction)
• Opponent reacts with Retreat
• React to Retreat with second Stupefy (or other might reduction)
• When second Stupefy resolves, unit has lethal damage and dies
• Retreat resolves but finds no owner, so no rune is exhausted

Nuances:

• Units with damage equal to or greater than their might are killed upon cleanup
• Stupefy itself deals no damage but reduces might, which can cause lethal damage
• Playing a permanent like Watcher starts a chain but doesn't pass priority; however, the on-play effect does allow reactions
• When Retreat resolves but the unit is already dead, no rune is exhausted because no "owner" is found