I don't think it's really reasonable to expect to skillmax anything off of collab invades, it's way too hard, but you have an option to try to save a few Piis at the cost of running nothing but the collab for a few weeks.
I had already decided that any Badpy i get would immediately go to Seph even before starting to farm the collab. I'm more annoyed at the stupidity of the luck-fest involved in his specific skill-up:
1) if you are lucky Gilgamesh invade
2) if you are lucky you drop the sword instead of Gilgamesh
3) if you are lucky you get a skill-up when you feed the sword
I won't try to calculate the actual probability of getting a skill-up from a run but it's damn close to ONE PERCENT, which is simply ridiculous.
I'll still farm the collab non-stop but it feels really frustrating....
The only saving grace is that the bgm is awesome, the enemies are awesome and, ironically enough, my Rinoa is already 2/3 on skillups :V