Throughout the game, the theme would be "Pyromancer = sees invisibility." Essentially, these enemies would have Burn My Eyes up at all times. I don't mean just any fire-themed enemy, I mean someone who is a dedicated spellcaster (usually holding a staff) and uses fire magic. There aren't a lot of them, but essentially every town has at least one, so it would be the occasional thing for invisibility characters to worry about. If the mouse over an enemy in the distance and see the Burn My Eyes status, time to come up with a new plan.
If you do manage to take out the town pyromancer, it will probably have a detrimental effect on your ability to purchase fire-based skillbooks, but it would come pretty close to giving you free reign of the town. Kill Arhu, and he can't help out anymore. If that's something you're okay with, then you can go back to using invisibility it pretty much the same way you can use it now, to steal everything with ease. After all, you did win a boss battle of sorts, so you kind of deserve it.
This is a brilliant idea and I recommend it heartily.
