The most basic implementation should look something like this:
- (void)rotateNode:(SKNode *)nodeA toFaceNode:(SKNode *)nodeB {
CGFloat angle = atan2f(nodeB.position.y - nodeA.position.y, nodeB.position.x - nodeA.position.x);
if (nodeA.zRotation < 0) {
nodeA.zRotation = nodeA.zRotation + M_PI * 2;
}
[nodeA runAction:[SKAction rotateToAngle:angle duration:0]];
}
It's important that you understand what is happening. Look up and read about atan2 (http://en.wikipedia.org/wiki/Atan2) and try to understand how the code above works.