Arcanist just doesn't give the 20% total damage debuff without ascendant. In other words, it doesn't work alone. For the Arcanist bonus effect to even work, you need ascendant for that And the ability to crit
Ascendant is basically an arcane dot (debuff over time) and the default duration is like 1 second but Arcanist extends the arcane debuff ascendant gives and only then, it applies the 20% total damage debuff per enemy when you crit. I tried an Arcanist build without ascendant but it doesn't work at all.
With hirlings, Arcanist effect is 2× more powerful because if hirling and main character attack, the debuff stacks like shock. You need ascendant and Arcanist on both characters for that to happen though and of course crit.
The reason why I chose weaken because the ascendant effect already gives 62.5% reduce resists when the arcane debuff is placed over the enemies and it syncs well with Arcanist 20% total damage per enemy. It might as well be ignore resist or maybe superior because of 25% more damage on the enemies the element is superior to. the effective mythic can work very well with this combo if you have the space which is awesome too.
Also, you can just use ignore resist for some Arcanist with ascendant builds. Remember, Arcanist doesn't function at all without ascendant.
Arcanist also doesn't work with elements set as elements set isn't an arcane Dot (debuff over time).