Hidden Conditions
There are several waves and variants of Saibamen that you'll encounter in three different locations. There are also Material Items that you can pick up per area. Just move from area to area until all Saibamen are defeated.
For the hidden conditions, you have to defeat all saibamen within 5 minutes. You have to defeat the ones in the mountain area, two waves in the Landing Pod area, another two waves in the Wasteland, and finally the last wave in the Mountain area again. After defeating all of them within 5 minutes, Nappa should appear. Defeat him to complete this mission.
I think it's random whether he appears. You might complete writhing the time limit and he might not show up.
The time isn't what makes Nappa appear. He appears when you have him in your party.
I beat all enemies within 3 minutes but nappa doesn't appear?