1/1944 Perfect Stage 7 RNG
1the ZPCI Assault has 2 attacks (Strike and Swat Fire) and the ZPCI Captain has 3 (Strike, Swat Fire and Swat Ray), with Swat Fire by far being the fastest one to get coming at you (probably around a second faster than Strike)
All enemies in this fight always have an equally likely chance of choosing any attack on any given turn so for the ZPCI Assault that's a 1 in 2 chance for choosing Swat Fire on any given turn and for the ZPCI Captain that's a 1 in 3 chance.
On top of that in order to save a turn you have to crit the ZPCI Captain at least on one of the two times you attack him. From personal experience this, combined with not missing any attacks throughout the fight, has roughly a 1 in 3 chance of happening (the crit on the ZPCI + not missing, that is).
Finally, after the fight there are 2 possible drops that can occur, each having a 50% chance. With the Assassin Build I'm using you need the ZPCI to realistically continue the run so getting that is also a 1 in 2 chance.
In this run, I got all 6 possible swat fires (2 from the ZPCI Assault and 4 from the captain), a crit on the Captain, no misses and the correct item drop after the battle. Using my 1/3 estimate (which is probably overestimating the likelihood) this gives a total
1/(2*2*3*3*3*3) * 1/3 * 1/2 = 1/1944 chance (0.051%) of this happening.
Needless to say this Stage 7 segment I got in this run may be hard to beat :)