Need Help Advancing A monster

[PrinceOfThieves]

I'm having a bit of trouble advancing a creature under Pathfinder rules. Its originally a 6HD, CR 10 creature that I need to advance to 20 HD. Can anyone explain how I determine what all to increase & by how much?

[LansXero]

In 3.5 you take the type descriptions and work off that; I dont think PF should be too different. Which creature is it btw?

[Acanous]

The Type of monster it is (Fey, outsider, dragon, magical beast, vermin, animal, construct, ooze, undead, plant, humanoid, aberration) tells you what kind of hit die to use. Every Two HD in pathfinder, they get a feat. Every four, they get an ability increase.
If you wish to bump this thing's CR to 20, you actually get 13 bonus HD you can give it, because every 3 HD you add decreases CR by 1.

[TheOOB]

With that severe of an HD increase, you may have to redesign the monster from the ground up. While the rules for giving extra HD are pretty simple(they act a lot like NPC class levels), it is unlikely the monster will have the abilities neccesary to make it a challenge at it's new hit dice.

Unfortunately, there is little concrete info I can give. Making monsters in D&D 3(.5) is more art than science.

[Feytalist]

Urpriest's Monster Guide (http://www.giantitp.com/forums/showthread.php?t=207928) may help you out a bit here. Just substitute whatever rule changes PF made and you should be fine.