Right, my mistake, the guarded conjuration cannot be attached to anything. I read too quickly.
Also, I just noticed that Guard dog would cost the same amount of SPB then [mwcard=MWSTX2FFC04]Cerberus[/mwcard] for a Dark mage. It seems to me that Guard Dog is a better investment than Cerberus.
With his vigilant trait, he basically gains a Doublestrike every turn (not exactly, but close enough). While Cerberus cannot be pushed, he can be teleported, Knocked Down, Incapacitated, dazed, etc. So, the unmovable trait doesn't perform miracles.
So, in the end, for 9 more mana, all you effectively get is Tough -3 (useful, but not necessary), Piercing +1 and one more strike (maybe). Not the best deal you can have if you ask me.
Go Guard dog! I feel like I'm going to have to make some space for him in a lot my spellbooks.