Use a Mystery Verse on Tho'at 1, use a Mystery Transformation on Tho'at 2. Call it a day.
Oakensorc is great.
I also use a modified build from The Gameroom, the "two button solo arcanist" build here:https://www.thegameroom.tv/arcanist-builds Hexos, Briarheart, Slimecraw and Oakensoul with a lightning staff. I run Flail, Beam, a heal, Wall of Elements, Inspired Scholarship, and Tide King's Gaze. The only hard piece there to get is Oakensoul, the rest is overland, from Traders, or from one of the easiest vet dungeons in the game.
I only use my ultimate on the bosses, or for 'oh crap' moments, because the build uses the red CP point "Strategic Reserve - Gain 30 Health Recovery for every 10 Ultimate you he." (The build recommends Gibbering Shelter for the Ultimate, and only ever used for 'oh crap,' but I swapped in Tide King to burn down bosses faster. Tomefoolery is a great verse for this build, because you can refill your ulti really fast.)
Definitely he a tank companion. If going into Arc 2+, I highly recommend Azander, because for some reason, he's the least likely to lose taunt on the mauraders. I slot Puncture as his first skill, and his class taunt as the 4th, and he does just fine.