Like Teeth said, try to position yourself to the side of the horse and aim at the legs. You get a wider viable target that way, and if the horse starts turning it's less likely to maneuver your weapon into the rider's block. If the guy starts riding off away from you you may be able to hit him in the back as he goes, too.
If you're super crafty you could even try to get behind him while his horse is rearing up to get past his block. If he's paying attention that's usually not worth trying unless you've got teammates around for him to pay attention to, though, because forcefield is strong.
In my experience, a bamboo spear is a fine and solid enough weapon to carry on its own, since you have it masterworked. Just remember not to rely too much on the overhead! OH YEAH THEY BUFFED THE OVERHEAD DAMAGE!