Just to help out some people:
You can buy the Battle Mode for 1500, there is an shopkeeper in the train, in the room right after the dark one. You can't miss him. There is a save spot there, and an mage. Talk to the mage for Battle Mode.
The good news is Baron Brixius is the easiest fight in the game for a Biological. His main weapon is the infamous Tick Tock, but it requires focus and regaining it through his Deep Burning effect costs him a lot of health. So equip Disrupt four times, leave the rest of your action bar empty, and watch him tear himself apart. This fight is so simple you may wish to reload it until he drops the Unholy Runeblade. You can't use it quite yet but it will carry you clear through to the Mayor.