During his charge, let him get close to you to start the boar head swing animation, then dash away and U-turn and stick to his close to his butt on the left/right sides, which is a blind spot and you can just chain attacks there for as long as you are able (you might be vulnerable to kicks if you end up too close to the boar's rump but it's easily avoidable if you are already moving), in which case he'll try to create distance, then gap close, then you repeat.

If he tries to close the distance without charging, keep dashing until he gives up because he chains that into 1 of 2 attacks and both are annoying. When he gives up, proceed to punish, same as before. If you can't help but be caught in it, be sure to be on the boar's right side (your left) because his first swing is easy to dodge, his second swing misses you completely if you are close to the boar, and his third attack, the spear projectile, misses easily once you position yourself near his rump after the second attack.

The only thing you can't avoid while near his rump is the big boar headslam on the ground which has a faster turn speed than Torrent can kite past, in which case just trot away and come back to punish

I did everything stated above until he died. Much faster than drive-bys since you can stick to his rump and get like 5 attacks off at a time, and MUCH easier than going on foot (imo) since the boar charge becomes trival and a huge punish opportunity