I would put points into the passive that increase spell penetration, elemental damage and regen (I think those are in the 3 different groups) You also want to invest points into the constellation that gives a 12% boost to Spell Critical (IIRC< the same one as Spell Penetration)
@Nestor, @Lennz:
I would not put points into Spell Penetration (Spell Erosion). This champion passive has been and afaik is still bugged - see this thread. Appartently 51 points in Spell Erosion will only net around 2 % damage increase.
Having a sharpened weapon or the Apprentice? mundus stone is much more useful to get spell penetration.
I would put points into either Thaumaturge (if you use attacks dealing magic damage - i. e. Dark Flare, Puncturing Sweep).
If you mainly use elemental damage (e. g. Impulse, Crushing Shock from the Detruction Staff skill line) then put points into Elemental Experts. These will give you a much larger damage increase, even if you dont have many points yet you can soon reach a bonus of 10 % or more. (I am currently at around 200 champion points and am getting a 15 % damage increase from Thaumaturge).
Another note: Thaumaturge is also bugged - but to your advantage. It will also increase your healing done. This is not often talked about, I think because ppl dont want it to get repaired ... but I think its good to know that.
As Nestor said above, in any case be sure to put 30 points into the Apprentice to get the 12 % critical strike bonus.
@Nestor, @Lennz:
I would not put points into Spell Penetration (Spell Erosion). This champion passive has been and afaik is still bugged - see this thread. Appartently 51 points in Spell Erosion will only net around 2 % damage increase.