Trouble orienting a GO using transform.up

I am coding the AI for my airplane-like enemies. Due to their nature as aircraft, they have a constant forward speed and would change heading by rotating their up vector in the direction of the enemy, then pitching up until the nose is pointing at the enemy. I did this using the game logic that follows:

First, I found the target position with respect to the plane (InverseTransformPoint), then I ignored the z component to collapse the problem to a two dimensional one. Then I used Atan2 to get the rotation angle from the forward vector of the plane to the target. Finally, I send it to the aircraft script and LERPed the angle. The aircraft "rolls" in its FixedUpdate. The script looks like:

In the AI choice switch statement:

 case 3:
     objective = AIObjective.attack;
     primaryTarget = SelectNearbyAllies();
     desiredPitch = -.5;
     if (!primaryTarget) {
         objective = AIObjective.straight;
         desiredPitch = 0;
     }
 break;

Later, set the target as the enemy GO:

 case AIObjective.attack:
     targetPoint = primaryTarget.transform.position;
 break;

Then find the proper angle:

 var tempVec: Vector3= transform.InverseTransformPoint(targetPoint);
 desiredRoll = (Mathf.Atan2(tempVec.y,tempVec.x) * Mathf.Rad2Deg)+90;
 if (desiredRoll < 0) desiredRoll +=360;
 if (tempVec.x*tempVec.x + tempVec.y*tempVec.y < .001) desiredRoll = 0;

 SendRoll(desiredRoll);
 SendPitch(desiredPitch);

And in the aircraft script:

 var newRoll = Mathf.MoveTowardsAngle(0, -roll - transform.eulerAngles.z, rollSpeed*Time.deltaTime);
 if (newRoll != 0) transform.RotateAround(transform.position,transform.forward, newRoll);

 if (pitch != 0) transform.RotateAround(transform.position,transform.right,pitch*Time.deltaTime);

Any advice would be appreciated. Please remember that I must send only the amount to be rolled and the necessary pitch to the aircraft script (without a serious rewrite), and that the aircraft can only pitch from transform.forward towards transform up (along that 2D plane). Thanks!

EDIT:

I've gotten close with the following:

 if (!primaryTarget) return;
 var tempVec: Vector3 = primaryTarget.transform.position - transform.position;
 tempVec = tempVec.normalized;
 var upVec: Vector3 = Vector3.Cross(transform.forward, tempVec);
 upVec = upVec.normalized;
 var combinedVec: Vector3 = Vector3.Cross(tempVec,upVec);
 combinedVec = combinedVec.normalized;
 Debug.DrawLine(transform.position, transform.position + tempVec *10, Color.blue);
 Debug.DrawLine(transform.position, transform.position + upVec *10, Color.red);    
 Debug.DrawLine(transform.position, transform.position + combinedVec *10, Color.yellow);

 var dR: float = Mathf.Atan2(combinedVec.z,combinedVec.y) * Mathf.Rad2Deg;
 if (dR < 0) dR +=360;
 dR = 360-dR;
 Debug.Log(dR);

 SendRoll(dR);

Now my problem is that I can't pitch towards the target... once I do the plane spins erratically or not at all... Once again, help would be appreciated!