How do you fight a Guardian Scout?

For regular swing/stab attacks, just flurry rush them. Right before it attacks, it'll bring its weapon back and rotate it around the wrist joint.

When the scout does its spinning charge attack, get behind a pillar (a cryonis pillar also works) and wait for the scout to crash into it. It will then be stunned and you can get a lot of attacks off.

When the scout does its spinning laser attack, take out your paraglider and use the updraft to get close to the scout, then slam attack after it's finished. Alternatively, you can also send some arrows while bullet time's available.

When the scout is almost dead, it'll start charging its laser for a series of consecutive shots. If you're close enough, try to kill it before it can let any off, but if not, wait until it starts shooting and run around the scout in circles; the lasers will all miss.

You can also shoot an arrow into its eye to stun it. Additionally, Ancient weapons or armour will be extra effective.

Later game, once you have better weapons, more stamina, and don't want to deal with all of that, equip a heavy, two-handed Ancient weapon (eg an Ancient Battleax+), a high damage multishot bow, and Ancient armour. You may also want to eat some food with strength buffs. Then, do your charge attack and spin around the scout in circles. You will very easily get the scout to critical health, and when it starts charging its laser, shoot it in the eye and finish it off with another charge attack. You should be able to defeat any scout this way in half a minute or less.