Disclaimer: my hatred of geologists is purely theatrical, but if I did have to kill one for some reason, it would be very easy.
I’d brandish my obsidian knife at them and they’d be compelled to approach. “That’s very cool,” they’d say, confident in their superior strength and endurance from all the rocks they carry around at all times. They’d shower me with very interesting facts about obsidian and hover just out of range of the cutting edge, waiting for me to exhaust myself. “But as it is volcanic glass, it’s very fragile, you see, and isn’t well-suited for use as a weap—” and then I’d hit them with the wooden baseball bat in my other hand, which they would not have noticed because geologists can only see rocks and minerals.