So he seems to randomly pick a side and charge through the middle. Ive seen N/S, E/W, NW/SE and NE/SW. If youre close to the side hes charging from you can see him and the charge marker is immediately visible. If youre away from his starting point, the charge marker appears more or less as he starts charging. If you do this with a set team of friends, next time plan to spread out and each of you do a specific check(ill do N/NE, you do E/SE, etc). One of you will spot him right away, and with warning everyone will have time to avoid.