Boromir of the White Tower

Reading this^ will certainly give you some basic infortmation about modding.

 

Anyway, go to experiencelevels.ini, and find Aragorns' experiencelevels (or copy Aragorn's and make a new one if you want to make Isildur level up faster)

You'll find this:

ExperienceLevel    AragornLevel1
    TargetNames                        =    GondorAragorn    
    RequiredExperience                    =    1    
    ExperienceAward                        =    ARAGORN_LVL1_EXP_AWARD
    Rank                            =    1
    SelectionDecal
        Texture                        =    decal_hero_good
        Style                        =    SHADOW_ALPHA_DECAL
        OpacityMin                    =    50%
        OpacityMax                    =    100%
        MinRadius                    =    40
        MaxRadius                    =    200
        MaxSelectedUnits                =    40
    End    
End  

Simply add "GondorIsildur" behind "GondorAragorn". Do the same for experiencelevels 2-10. You're done.

It didn't work...

I tried making Isildur's own leveling system, but he still comes out at level 10. I'm out of ideas.

You can make a fairly basic version of Sauron with the epic mace knockback powers by adding MordorSauron to the Mordor player template in ini.big...