From a min/max perspective I'm not sure. I just never cared for the Tonitrus, mostly because of its low durability.

The interesting thing about a pure arcane build is that you often don't even need to use your weapon because the hunter tools deal so much damage. For a ton of bosses you can just create five blood bullets, heal back up, use the executioner's glove twice, and repeat until the boss is dead.

Check it out sometime! There's also a ton of useful information in this guide: https://www.bloodborne-wiki.com/2017/04/the-arcanist-20.html