[Agramer]

1)Such "Monster wolf" would have Combat reflexes most likely

2)Wolf is already attacking,he "ambushed party" so he doesnt roll anything.
-sequence is established Wolf is first,than are PCs

3)Since PCs are entering his den and expecting combat they cant be totally
surprised,so in your example theyre Partially surprised.Since Wolf is
ambusher he acts first while PCs are mentally stunned.
When PCs turn comes(when wolf is finished...maybe he already attacked them or is still running toward them,depends on your setup) Each PC must make "Do nothing manoeuvre" and rolls vs Will to get out of mental stun(Combat ref give +6 to that roll).

Than ,its Wolfs turn again....

After wolf is done is PC turn...those who snaped out of Mental stun last turn can act normally,those that didnt now roll vs Will+1( +1 to Will on 2nd turn,+2 on 3rd,+3 on 4th...etc since this is partial surprise).


4) Only time when you roll initiative for both Wolf and PCs would be if they didnt noticed each other and have blundered on each other.
So called "encounter" when neither side expected encounter...than you roll initiative

Basic p 393.