These guys are quite common, and they often serve as backup for their distant cousins, the Gold Ghosts. They are fairly strong but slow, so if one is equipped with the proper ghost hunting equipment they shouldn't pose much of a threat.
Attack Pattern: Purple Punchers materialize from the darkness in unlit rooms. They approach their enemies, and then wind up for a very big punch.
Basic Fighting Strategies: These guys are a bit dangerous because of their long-range punches, but they take quite a while to wind up before punching, so it's pretty easy to dodge them as long as you aren't surrounded by ghosts. Dodge the punch, then spin around and hit them with the flashlight.
Advanced Strategies: An important tip for catching all ghosts is to rapidly and repeatedly press in the opposite direction that the ghost is moving using both the control stick and the yellow c-stick while you're sucking it up with the Poltergust.
Because of their quick, long-range punches, catching multiple Purple Punchers at once is pretty hard. You can try to move to one end of the room, and then spin around and suck them up before they try to punch you, but it will be tricky unless they are close together.