Well met Tarsiz and welcome! Good luck with your Assassin. You might find that in BGEE v2.0 Assassins underwent a serious (and in my opinion questionable) nerf: the game now allows foes a saving throw to completely negate the poison effect. Hopefully this doesn't spoil the fun for you. Enjoy your run!
Gate70 congrats! See you in Amn 
Wynter, LN Human Invoker, final BG1 update
Besides procuring a DEX tome, Wynter had more business to take care of in the city, such as adding tomes of INT (Ramazith stunned with Wand of Paralyzation) and WIS (Chantalas Ulbright pickpocketed by ferret) to his collection, finding and selling two of ol' Balduran's items, slaying Doppelgangers, and nearly getting himself killed by an Ogre Mage and its pet Carrion Crawlers. This last feat was quite astonishing. One moment the Invoker was standing between his soon-to-be-foes, invisible and getting ready for battle with the quaffing of a potion of freedom,
and the next the Ogre Mage was aware of Wynter standing next to him. I really have no idea what had made Wynter become visible, or otherwise prompted the dialogue with the Ogre Mage. Maybe my mouse did something I didn't want it to (I've noticed it behaving strangely as of late). The consequences were very serious. Wynter tried to run, he was still hasted but didn't enjoy warp speed with the combined effects of the boots and an oil of speed, due to the potion of freedom. However, the Ogre's Horror caught up with him before he could quaff a potion of magic shielding (he had a muddy aura). Miraculously he saved against a Charm, survived two Magic Missiles, and avoided most of the Carrion Crawlers' attacks.
Once back in control of himself, he immediately drank an invisibility potion, then healed, buffed, and finished his enemies with a Cloudkill.
Cloudkill was also Wynter's opening move at the Iron Throne. It killed not-yet-shapeshifted Emissary Tar, and injured most of the others. Knowing himself outnumbered, and aware that his spell buffs weren't going to last long enough, Wynter decided not to fight all his enemies at the same time. He drew Naaman, Diyab, Aasim, and Alai downstairs one after the other, and made liberal use of his wands (Fire, Paralyzation) to defeat them and pick up a coveted Minor Sequencer scroll amongst other loot. He approached Thaldorn invisibly, and quaffed an invisibility potion when he was done talking, as he heard a voice from the shadows sayin "Sorry, friend, but you and I've got a date, down under." (One of the Shennaras had located Wynter.)
Before traveling to Candlekeep to meet the Iron Throne leaders, Wynter undertook a little excursion to an ice island somewhere in the Trackless Sea. Invisibility, and spell and potion buffs allowed him navigate the dungeon without having to fear its traps. Wynter made sure he covered the whole floor, triggering the traps while he could. He went straight for Dezkiel but couldn't find him. My characters don't often go to the ice island, but for Wynter it was the only place where he could get his hands on a Stoneskin scroll. I had him fight the other wizards, playing fair against the first three (Andris, Beyn, Marcellus). Wynter engaged them fully buffed, which at this stage of the game meant Shield, PfE, Blur, MI, MGoI, II, and had them waste their spells on a summoned Skeleton and wand summons. Of course Andris's Remove Magic was successful, whereas Wynter's attempts at stripping the opposition's protections failed, so the latter ended up staying out of his enemies' sight. When Andris seemed out of spells, and Wynter approached, the former still had a Flame Arrow that burnt away half of Wynter's HPs, but that would be Andris's last feat. Wynter wand-scorchered his foe to death.
Andris dropped a Remove Magic scroll, a nice find for Wynter who hadn't encountered that scroll before. (I believe Davaeorn is supposed to have one, but I must have missed it this time.) As I couldn't be bothered with fighting the other mages fair and sqaure, I had Wynter read a green PfMagic scroll and just cruise the dungeon with his wands and the occasional scroll-cast spell. This may not bode well for BG2: I dislike repeated resting, and I tend to lose interest in sophisticated buffing/stripping chessgames and much prefer to just get to business asap, even with my mages.
With all mages down, and still no sign of Dezkiel, I consoled the latter into the game. Wynter then had him fight wand summons while waiting for his foe's protections to expire. Again, this is starting to become repetitive, the Wand of Fire finished the job.
(I later discovered that BG1 Unfinished Business's Ice Island Level Two Restoration was the reason Dezkiel hadn't spawned; he spawns on the island's restored second level.)
Back in Baldur's Gate another thrilling experience awaited the Invoker. Ferret failed to pickpocket Resar's Fireshield: Blue scroll, the last scroll Wynter desired for a final scribing session, so the Invoker ended up slaying the Haalruan with MMMs and a Skull Trap.
Hostile Thieves were Cloudkilled but Alatos had a nasty surprise for Wynter on the way out
In and below Candlekeep, Wynter limited himself to plundering bookshelves and vaults, leaving the Iron Throne leaders and Prat & Co alone.
Wynter returned to Baldur's Gate to see Slythe waste his invisibility potions on wand-summoned Hobgoblins. The Stoneskinned wizard lured the assassin through the Blushing Mermaid onto the streets, where a wand-scorcher killed a courtesan for a late, irrelevant reputation drop, and back to the Undercellars, where he finished his foe with a Magic Missile right in front of a second Slythe and Krystin.
(Due to a bug the two respawn if Charname leaves the Undercellars and returns.)
I was worried about the Ducal Palace but Wynter had an easy time there. He had made use of an aTweaks ability that allows wizards as of level 9 to scribe scrolls of memorized spells (costs gold though and the memorized spell is used up for that day). He summoned five Skeletons (from scrolls), buffed with the usual potions (violet, agility, mind focusing, fortitude, magic protection, defense, power, speed, regen), and with several spells (Stoneskin, MI, Blur, Fireshields, II), Hasted the Skeletons, cast Invisibility on Liia, and prevailed.
Both Dukes survived; Belt received some healing from Wynter after the Greater Doppelganger Mage had injured him and others with a Fireball. Sarevok did not reach Belt, and thus the story continued in the Undercity.
Wynter rested and, with the violet potion still effective, rebuffed with the same potions as at the Ducal Palace. He had heavily buffed Angelo shoot his wad at summons, before he fried the Flaming Fist commander with his wand. Semaj and Tazok were done in with Fireballs shot from a distance; Diarmid was wand-scorchered to death. Wynter killed the Skeleton Warriors with his staff while protected from Undead. His Boots of Speed combined with oils of speed ensured the wizard's movement advantage over Sarevok. That didn't keep him from taking a heavy blow after he proved overoptimistic in calculating his time for firing a Magic Missile at his foe,
but other than that there was never a doubt which Bhaalspawn was going to prevail, even though Wynter had to tap into his scrolls collection to submit his foe.
Thus Wynter became my first single-class mage and my first character in a nightmare run to no-reload his way through BG1. It hasn't been my most refined playthrough, with a few unnecessary close calls (Bandit Camp, Alatos, Ogre Mages, Sarevok), and with heavy reliance on items (wands, scrolls) although that was largely due to a lack of spells in large battles.