I recently had a friend of mine play a game in which this encounter occurred. Tel Trevura equiped with R5-P8 was getting fired upon by a Black Sun Ace with Cluster Missiles. The Black Sun Ace had only one health left when making this attack. After the first volley of attacks from Cluster Missiles, R5-P8 was rolled and the result was a hit which did the final damage to the Black Sun Ace.
I believe that since it's two attacks and R5-P8s card says "After defending" it would trigger after the first attack of Cluster Missile and destroy the Black Sun Ace. A few local players have debated whether the second attack from Cluster Missile should've been completed as part of a simultaneous rule or that the full attack of Cluster Missile should technically finish before R5-P8 can roll.
Thoughts?