Need a Giant - wizard CR 12

[Eretas]

I want to add some spell caster in Rise of the Runes lords and I don'T want to have only level 1 or 2 wizard.
So my problem is to donwgrade a standard Hill Giant and Stone Giant to give them 6 to 8 Spell caster level. But per default, the Giant automatically have 10HD for a Hill Giant and 12DH for a Stone Giant. With 6 to 8 wizard level, they will be way to strong and with 1 or 2 wizard level, they will be week wizard. So How can i reduce the base HD of the base Giants?

 

[Eretas]

Finally, I create two new race with lower HitDice.

 

[RavenX]

This sounds like something you'd want to do using the custom monster building rules the back of the bestiary, which you could do in hero lab, but you'll need to familiarize yourself with the rebuild rules first to keep your creatures balanced.