There are 3 different approaches I have to go against them with varying degrees of success.
The best and easiest way to handle them is to use a shield and Shield Bash them as they attack.
That will daze them for a while so you could counter.
Their heads should be lined up by then.
Note that if your shield is weak and about to break, a shield bash will not damage it.
Another way is to use the Skyward Strike on them.
You just have to slash in the orientation that their heads are in.
This one isn't too reliable as you have to anticipate their moves (and need some luck).
Probably the hardest way to deal with them is to fight "head-on."
When they're about to strike, their heads will be oriented in a certain way.
If you have a fast enough reaction time and close enough to attack, you could slash in the same orientation to kill them off.
A more cautious approach would dodge back first when they do attack and counter.
I don't remember what their orientations are after they attack but it will probably be in the same as when they attacked or horizontal.
You may have to experiment with that.
Probably the least successful way to deal with them would be to randomly attack them.
Their heads will mostly be not lined up so you won't be able to kill them that way.
Wait for them using the tips above to kill them.
gaming.stackexchange.com/users/8995/Jeff Mercado
They line up just before they're about to attack, as you've no doubt realised.
What you might not have noticed is that, if you don't hit all the heads, there's a small window while it's regenerating where you canfrantically swipe at the remaining head.
If you get it before the other heads grow back, it'll die just the same.
I think I've killed them the proper way once.
gaming.stackexchange.com/users/2734/Merus
Just use bombs.
They're a one-hit kill.
gaming.stackexchange.com/users/17669/HSO
Right before they strike, their heads line up.
One way to cut them off all at once is to be quick enough to go for a side slash just at this moment, but it's tricky.
Another way is a diagonal slash when the heads are in diagonal alignment.
You can also shield bash, although you need a good strong shield to make sure it doesn't break.
Hope that helped!
gaming.stackexchange.com/users/14061/Pwopa Grrl
Take out a head or two, then horizontal spin attack when they reform. works like a charm.
If you mess up, hit em again then take it from the top.
Don't bother trying to counter, it's not worth getting hit (I tried.)
gaming.stackexchange.com/users/18573/user18573
I have gotten killed by the Staldra four times tonight. I have no idea what I am doing wrong.   I know I have to hit all three heads in one stroke, but they never seem to line up so I can. If I wait to strike until they are in a diagonal line, it attacks me instead.   Is there some trick to this that I am missing?
How do I kill the Staldra?
gaming.stackexchange.com
<zelda-skyward-sword>
gaming.stackexchange.com/questions/40683
gaming.stackexchange.com/users/11502/Ash
You should use a shield and try the Skyward Strike. Try using bombs. The hardest way is to fight head-on. You could try a more cautious approach by dodging back when they attack.  While it seems to be a difficult feat, players list plenty of ways to deal with Stalkdra.
The community is offering some tactical approaches.