How do you get extra PA disks offline?

[Zorafim]

For the first PA, you just need to buy it in a store. What about the PAs that can't be bought in stores? How do you learn those?

[Tycho]

By getting S-rank in Missions you might randomly receive them from your NPC allies.

[Zorafim]

Let's say I can invite Leo. Since he has a spear and a sword, I'll guess he gives the spear and sword PA. I tried doing the first available mission available, but I can't seem to get an S-rank on it (I think I need to beat it in two minutes, which I just can't do). Could you suggest a mission that may be easier? Also, I'm assuming we're talking about extra mode, but is this true in story mode as well?