zelda-breath-of-the-wild
I was watching videos for tips to defeating Guardians in BotW and I found this video of guardian being defeated purely by reflecting damage back at itself. I know that you can do this with a well timed parry with any shield, but with the Guardian Shield it looks like you don't have to parry at all.
The only problem is, where can I get a Guardian Shield? I got one from a minor test of strength in a shrine, but it broke. Where can I get them reliably?
Find me a Guardian Shield
Just aim at the propeller "motor" bit. They don't take much damage from regular arrows, but two or three bomb arrows can destroy a propeller pretty reliably. I haven't experimented with other arrow types yet.
Here's a video: IGN
The Coliseum is the best place to get elemental weapons, and you don't need to unlock it as you seem to think. I've included a picture from this interactive map:
https://www.zeldadungeon.net/breath-of-the-wild-interactive-map/
You can see the Coliseum ruins near the middle of the map (marked by the bow and arrow icon). The Great Plateau is near the bottom middle as a reference point, and the stable shown on the left/top part is the Outskirt Stable. Just go into that Coliseum and have at it.
Warning about entering that area:
There is a Lynel at the bottom. If you aren't prepared to fight it, I recommend not using the front door (north side). Climb up over the walls or mountain, and enter from the top. There are 3 floors above the Lynel. Each floor contains enemies with frost, lightning, and fire weapons respectively.
Best Answer
According to this wiki, Guardian shields can be reliably found at both the Soh Kofi Shrine and the Pumaag Nitae Shrine after completing "A Minor Test of Strength".
Alternatively, this wiki lists the locations where Guardian Shields more generally can be found.
Guardian Shield: West Necluda, Lanayru Great Spring
Guardian Shield +: East Necluda, Faron Grasslands
Guardian Shield ++: Gerudo Highlands, Akkala Highlands
In all of these areas, the Guardian Shield can be obtained by killing Guardian Scouts.
As I mentioned in this answer to another Breath of the Wild question, Guardian Scouts will respawn after a Blood Moon event.
So strategically, I would visit the areas listed above, and make a point of targeting the Guardian Scouts, as it are these scouts which will drop the Guardian shields you are coveting. You can read about the strategy for efficiently defeating Guardian Scouts here. These enemies (both inside and outside of shrines) will respawn after a Blood Moon.