Historically speaking, timed fuses were far preferred to impact triggers for exactly this reason. Estimating the time of an arrow flight from the distance is a pretty straightforward affair, and if you're uncertain, you can just guess long; if the bomb sits on the ground for a few seconds before going off, it's no big deal.
For instance, the Wujing Zongyao, an 11th-century Chinese manual, suggests encasing the powder in a bag (of paper, cloth, or whatever's handy), attaching a fuse, and simply fixing the whole lot to the arrow and lighting it just before launch. Though less deadly than Greek fire or other impact-detonating schemes, it's far simpler and less likely to result in your troops lighting themselves on fire.