Level
Source GM Core pg. 117
For most creatures you build, their level depends on the level of the party that will encounter it. Look at other creatures you think are similar in power to yours to determine its level. Note that level represents a creature's combat ability, so a creature that's more social might have 3rd-level combat statistics and 6th-level skills, but it would remain a 3rd-level creature. Most such creatures are NPCs; for more information on this distinction and how to use it, see Non-Combat Level.
Some abilities are hard for PCs to deal with at low levels. For instance, creatures with an extreme AC at lower levels can make for frustrating foes that require exceptionally high rolls to hit. Natural invisibility or at-will invisibility as an innate spell should come at around 6th level, when PCs are more likely to prepare see the unseen in lower-rank spell slots, or 8th level, when some PCs get access to the truesight sight.
The tables in this chapter go up to 24th level, the highest-level extreme encounter a party might face.
Some abilities are hard for PCs to deal with at low levels. For instance, creatures with an extreme AC at lower levels can make for frustrating foes that require exceptionally high rolls to hit. Natural invisibility or at-will invisibility as an innate spell should come at around 6th level, when PCs are more likely to prepare see the unseen in lower-rank spell slots, or 8th level, when some PCs get access to the truesight sight.
The tables in this chapter go up to 24th level, the highest-level extreme encounter a party might face.