Thanks. There will definitely be more events with Nel
Regarding fighting the jellyfish:
Buy and learn the anti-paralysis potion from the alchemy shop.
As soon as you first step into the beach, you will get the first item: banastar.
This will allow crafting anti-paralysis potions which are very EFFECTIVE against paralysis attacks. Use one BEFORE the fight.
The jellyfish don't have enough MP to execute more than 2 paralysis attacks per battle, so you will eventually be able to beat them.