Bottomswell
Bottomswell is a damn annoying boss for sure. For starters since it is a "flying" type boss only long range characters can even attack it physically. This can be eliviated with the Long Range materia.
Bottomswell has two attacks that you sohuld be wary of. The first one is called Water Bubble which encases a character in a bubble of water that prevents that character from attacking. They can only be popped by magic attacks and if the entire party gets wet then it is game over
The second attack is called Big Wave and this is its big attack. Making matters worse is that it gets to use this skill as a final attack when you kill it so make sure the party's hp is well above 100.
As is normal with flying enemies it is invulnerable to earth but weak against wind, which do double damage. So if you have Choco/Mog use it heavily here. Keep bashing it with physical attacks with which ever characters can reach it and heal when you need to, use limit breaks as the become available. This battle should be relatively easy as long as you watch out for Big Wave.
You get a Power Wrist for defeating this monster.