It's not necessarily playing the monster "poorly" actually. For one, the monster is clearly outnumbered here, in which just concentrating on one guy would mean it has to stay near him, and therefore becomes an easy target. Constantly teleporting around to get little jabs in at different PC's will keep them guessing constantly.
Basically, think Nightcrawler from the X-Men. He didn't just teleport in and stand still next to someone and wail on them, he typically bamf'd all around the place striking from different angles to keep them off guard.
Secondly, it is a hound, right? Wolves are capable of taking down creatures much larger than themselves (such as Moose, Elk, even Bison) through what's called "Wolf Pack Tactics". Basically, it means that they encircle the prey and one side will attack the backside of the animal and then retreat when the animal turns. Then the other part of the group will attack now that it's attention is focused on the ones that just attacked it, at which point the animal turns again. Rinse and repeat.
In this case, it's the hound that's outnumbered, but it doesn't
need others to constantly switch attack angles and engage the other "side" of the party. If they all circle up, for example, it could attack on the side where the Defender isn't, and then when he tries to engage the hound teleport back to the
other side of the group where he just was and engage the squishies over there. By bouncing around like that it will keep the party off balance and won't allow them the time they need to set up complicated tactics and build a defensive line.
Of course, it also has the benefits of spreading damage around a bit and helping the party to survive the encounter, but they're not going to be thinking of that when they're constantly worried about where the next attack is coming from and who the next target is. And plus, it's not like you have to
tell them that you're trying to make sure they don't just wiped out.