Get out your shield. Even if it's a pot lid. Then use L to lock onto the guardian. While your shield out you press A to parry. If you press A at the right time, even a pot lid can send the laser right back in the face of the guardian. If you get the timing wrong, you will break your shield.
The point is, that if you have the skills to press A at the right time, you will you can take on any guardian no problem.