A Stab, A Slash, & Duck

Castille hurts it, and the thing snarls. "How DARE you!" It goes after Castille in the initiative count, so it slashes at him using its once-a-day smite good ability. And I rolled...a 3, with the mods that isn't enough, and its smite attack is wasted. Castille ducks, the rippling evil flying to one side. (I'm tired, I can't get more poetic than that really.)

I'm assuming that Kaspar is taking a 5-foot step either sideways or northwards. (5-foot steps never draw an Attack of Opportunity.) Because if he isn't, i.e. if he is still standing next to it, drinking a potion (a full-round action, but yes you can get a five-foot step with a full-round action) draws an AoO, and you want to give this thing as few shots at you as possible. So I am assuming that you step, drink, and are ready to roll.

However...

Yes, the door you came in is still open, or to be precise, there is no door there any more, it slid back into the stone.

So do you 5-foot step to the side (east), preparing to get some distance and bolt?

Or do you take that 5-foot step to the north, which next round puts you into a position for a 5-foot step to the west, which makes you flank the creature, which gives you sneak attack bonus damage if you hit.

And what is Castille doing? He begins the third round since the creature was summoned. (Make a Knowledge Arcana check.) Castille goes, then this beast, then Kaspar.