You want to boost healing up to learn the regeneration spell; it will be incredibly useful in one specific boss fight. (You get this spell in the gardens.)

A combination of Ghost Form potions and Fire magic actually works really well, as long as you are careful not to burn yourself. Remember, while in Ghost Form, you can still hit (and be hit) with magic.

The best skills endgame are Alteration and Protection, but you don't get them until late. You have enough time to boost Alteration to a decent level, but you should save the respec orb until you get the final Protection spell.

As for robes, the main reason to boost the skill is that one of the three main types of robes will do the following:
a. Allow you to cast spells of an element faster
b. Reduce the amount of essence you use when casting a spell
c. Make it so that taking a hit will not interrupt the spell you're trying to cast. This works for all spells.

Endgame, a combination of Fire magic, a fire staff, fire robes of concentration, and Alteration magic to improve the staff and robes works *really* well offensively.

Maybe I'll load up my Hard++ save and take a picture of my endgame skill point allocation.

Of note, I have not played with the dagger skill you mention, but it sounds useful. It is worth experimenting with it to see what you can do with it; can you cast spells without breaking the invisibility effect?

Haven't tried casting spells, but for exploration and avoiding enemies, that invisible thing is priceless!

Have you tried using levers or similar devices? (The downsides of ghost form are that you can't hit with physical attacks or use levers.)

As I said, it is a good idea to experiment with the ability and determine what its strengths and limitations are.

Here is, attached, my endgame skill build for my pure mage Hard++ playthrough.

Of note:

Reaching level 16 is feasible; reaching higher levels really isn't as of the final save.

There is one area after this, from which you can't escape (barring a glitch, and I think there is one), and in which you realistically could gain another level, but there is a time constraint at that point.

I managed this without using a respec, but I personally recommend saving your respec for when you get the spell that requires 14 Protection and otherwise not worry about saving points for it. (You will want alteration, however; it's how I got my casting speed bonus up to 37% rather than only 30%.)

Thanks for the link!

I tried a bit the ninja smoke thing from the dagger, it is great, but traps still hit you. I didn't find a lever to use but i can use doors while it is active. To me it seems exactly the same as the ghost potion, which also isn't trap-proof. The dagger skill is much shorter in duration, too, but key for a critical getaway or a lightning fast exploration in crammed places.

Now i am at the boss of the gardens and i need to finish a side quest. Fire saved me, really. From Heat-shield against pests, to fireball against packs, to the improved fireball combined with the frost elemental pet, that was exactly the boost i was looking for. Will try to go mage later! Cheers!