I just captured the undead princess at level 17.

[Fast]

If you cast banish on her (light damage) you can capture her but you may need a lot of phoenix downs.(i used 5)

Unstack both.
banish with the paleberry over and over(use ethers)

when both lann and reynn are dead use phoenix down.
You can use Howl to reduce her chance of hitting if you have it.

[ネチガワラ]

Nice!! I never try to do it :D

[Scarlet Crusade]

It's the same with Goblin Princess, you can also capture her obscenely early. The problem with this on either of them though is they give you hella EXP and you're going to be way more powerful than you should be. They really should have not given EXP when you imprism.

[Fast]

Originally posted by Scarlet Crusade:

It's the same with Goblin Princess, you can also capture her obscenely early. The problem with this on either of them though is they give you hella EXP and you're going to be way more powerful than you should be. They really should have not given EXP when you imprism.

its a built in feature like other FF games you can do things earlier than you should
or gain large amounts of XP

Like the cactaur in FF15 you can kill at the very start of the game and bump your XP.

Quite frankly the power scaling is on point in this game and even with the small 3 level boost per prism its still challenging.