Wipqozn has it right. Use the boomerang.
However until you get the boomerang, you should be able to fight them with deku sticks. Also using the magical charge attack seems to work.
In the second room there are several jellyfish called Biri. Keep away
from them. They are invulnerable to Deku Seeds from your Slingshot and
give you a shock if you try to kill them with your sword. However,
remember the Spin Attack you learned? Utilize that attack on them so
that they won't injure you. If you hold the attack long enough, it
will grow more powerful and you can release a Super Spin Attack that
reaches further. However, it takes magic to use this attack. Some
enemies drop Magic Jars you can use to keep your magic meter filled.
If you have deku sticks with you, you can use them to fight the Biri
without getting shocked.