Help with instantiation and rotation

You may be able to simply drive transform.forward (or transform.up when in 2D) equal to your movement vector, such as.

slashInstance.transform.up = vectorToPlayer;

NOTE: you must get slashInstance back from the Instantiate() call.

Otherwise, use Mathf.Atan2() to derive heading from cartesian coordinates:

Sine/Cosine/Atan2 (sin/cos/atan2) and rotational familiarity and conventions