So, I finished the game with Elementalist.
I tried the Staff Summoner and oh boy, how underwhelming I found it to be.
Throughout most of the game, the "basic elements" served me well enough.
After more or less lvl 20+, I felt the need for some change. So, I kept my lvl 4 Lightning, but I changed my Fireball for Combustion, and my Blizzard for Disintegrate, and I never looked back ever since.
The Ark is hard, yes but... Surprisingly I found it easier with my Elementalist than with the Summoner that I tried later.
I found out that the Summoner require too many skills to reach the lvl 18 golem; the Cooldown is insanely long and the mana that it drinks is no joke.
The Iron Golem does poor damage and not exactly a super tank, it doesnt help for much than 2 or 3 spiders individually, and barely nothing when they are in group.
So, from my experience, zapping them and running is better than using golems; just use healing potions. After that, the Ark became fairly easy.
I did got the Sparkling lvl 1 thou, as those are invaluable to deal with those damn Steel Wizards. It's still possible to deal with them without putting yourself in serious trouble withiut it, but the sparkling makes it so much easier, and particularly on IM you dont wanna take the unnecessary risk.
So for my "Elementalist" lvl 24, I have Zap 4, Mage Armor I believe was 2 or 3 (when I reseted I had to lower 1 lvl, Im not sure I bought it up back to 3 afterwards), Staff 4, Mana Surge 3, Combustion 3, Disintegrate 3, Arcanist 3, Gate 2. Im doing it from memory thou, I might be wrong here, but I guess it was it.
My traits were Str 2, End I believe was 4, Agi 4, Int 8 or 9, Awa 2, Per was 4. Im not exactly sure, but I believe it was it.
With these configurations, I went smoothly throughout the entire game.
Like I said, Summoner for me was paradoxically a lot harder.