As the Guardian charges its laser, the frequency of the music and circles on the lock-on guide on you will appear more frequently and condense into a single dot. A second after this, the Guardian will fire the laser.
Here's the visual indicator of when it’s immediately about to fire
The targeting indicator will also disappear.
If you're close up, this is pretty much exactly when you parry. At much larger ranges, however, the laser has a longer travel time. If you're at these longer ranges, you're going to want to parry about half a second before it looks like the laser'll reach you.
Some more general tips
- You can sprint and lasers won’t hit you, provided you’re sprinting horizontally enough (the lock on target will be inaccurate)
- The Ancient Shield will deflect Guardian lasers automatically, provided you’re holding it up
- Going behind cover will end the lock-on, at least until the Guardian has you in its sights again
Watch this YouTube video for more information.