The video game rule of vulnerability comes into play here: Hit 'em in the glowy bits .
You can never go wrong hitting anything that glows.
In this case that means slashing horizontally to cut the blue lines between the "blocks" that make up the body then stabbing the glowing eye.
Or just arrow the eye if you have the bow.
The same rule of thumb applies to Armos and Bombos as well.
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If you can break enough of its segments to bring it down to ground level, you can kill it by stabbing it in the eye.
(ow!)
Otherwise, you can put an arrow in its eye (Oblig.
"I used to be a Beamos, until I took an arrow to my eye" joke) if you've already found the Bow.
From IGN's Wiki guide : Slicing the Beamos's different sections vertically (separated by the blue running line across) will bring its head to the ground.
A thrust into the eye will finish it off.
You can also keep out of its reach and then shoot it in the "eye" with an arrow once the Bow is obtained.
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You can also dizzy them first by blowing on their head with your blow gun
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I always took them out with horizontal slashes, or spins.2x, then stab the eye.
If he's on you, run a fair distance away then try again.
Strangely enough, you can stun them with a perfect shield bash if you have a wooden shield or sacred shield.
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Once you have the bow , you can kill them from a distance with a single shot to the "eye".
This is especially helpful on the Sandship when trying to defeat the two Beamos in the big key room .
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I'm wandering around, lost, in Lanayru Mining Facility.  As I was blundering about, I took a (horizontal) swipe at a Beamos and surprisingly, did some damage.  I've also noticed that a targeting arrow appears when I get close.   Can I kill these guys?   If so, how?
How do I defeat Beamos?
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<zelda-skyward-sword>
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You can put an arrow in its eye or break enough segments to bring it down to eye level. Slicing its sections vertically will bring its head to the ground. It seems that there are two common ways to deal with Beamos. There is a one shot strategy and a more time consuming one.
The required strategy to defeat Beamos is detailed.