What is the level based DC for something with a level of -1? I saw that AoN uses 13 to set the Recall Knowledge DCs, but I don't think the table provides a DC for level -1. And a note in the Beginner Box says the DC for a level -1 creature is the same as a level 0 creature.
At that level - just pick something. Use the value from level 1. Apply the easy modification to the DC if wanted.
I'm not sure why it would make all that much difference. Is there something specific you are looking at?
The DC by level table reads:
0. 14
1. 15
2. 16
3. 18
4. 19
So based on that scaling, it would be one or possibly two lower than the level Ero DC. So, probably 13, but you could go as low as 12.
Formula is:
Level DC: 14 + level*4/3 (rounded down)
So a 12.
Formula is:
Level DC: 14 + level*4/3 (rounded down)
So a 12.
The formula changes after level 20, though. And level -1 has some weird rules exceptions already. An elite adjustment turns such a creature into a level 1 creature. Furthermore, the attack and save modifiers are the same for level 0 and level -1 creatures, though almost everything else is different. So it's entirely possible that a DC based on level -1 was meant to follow different rules, but the devs forgot to say. And I imagine it hasn't been brought up by the devs and players very often, since Archives of Nethys just tells you it's 13 and I only noticed by accident that the table says nothing.