Here's a reliable way to get the herbs in New Austin with Arthur in Chapter 2. There are four you need to complete Herbalist 9. Using the buggy you can get all four in one trip. The only impossible challenge (for Chapter 2) looks like Bandit 6 and up. The horse fence only becomes available in Chapter 3. For Sharpshooter 6 you need the Rolling Block Rifle, which you only get in the final part of Chapter 2. It's also possible to get that rifle early using the companion glitch (which is difficult).

The AI voice was trained using ten minutes of ingame dialogue.

0:00 Introduction
0:47 The covered wagon
2:17 Taking the buggy
3:46 Crossing the river
7:25 Wild Feverfew
10:20 Red Sage
11:45 Desert Sage
12:50 Blackcurrant
15:00 Crossing Great Plains
18:00 Selling wagon