Decent Grinding in FFIV

This is available after the Cave of Monsters/Eidolons in the Underworld is accessible.

First, you may not want to attempt this until level 30, maybe. Make Sure Rosa has Float, it guards against the damaging areas on the floor, it also helps defend against the Arachne enemy, which is what you want to fight. Walk around the cave until you run into a Summoner enemy, cast Mini on him ASAP, and ignore him the rest of the battle. Let him summon up any kind of monster except for Goblines, they give crap EXP and Gil. Arachne is the best enemy for him to summon since you have Float active, the only thing Arachne can use is Earthquake, so aside from the persistent Mini-Summoner attacking you with 1 damage in the background, you're completely safe. Keep this up for a while and you'll accumulate tons of EXP and Gil, five Arachnes could give up around 20,000 EXP and lots of Gil, which isn't very important around this point. Fast, easy and totally cheap, but it works. When you think you're good, just kill off the summoner and reap the benefits.

This is very useful to use before heading up to the Moon, the enemies there are pretty difficult, even at level 40.

That's a pretty good idea for lower levels, plus your characters will be strong enough to fight Asura and Leviathan without any difficulties.

I've been grinding on the Moon and fighting the monsters in the area right before you reach Zeromus. Each monster gives a shit ton of EXP and Gil so I'm fucking rich right now. I think I've got like over two million bucks so I bought 99 of almost every item so I can heal out of battle without wasting MP and I have so many remedies that status effects don't stand a chance to me.

^Did that, too. xD

A side note, Thunder Dragon is a good enemy to fight if the Summoner calls him, he offers more EXP than the Arachne, but can annihilate Kain in two shots, everyone else takes like, 200 damage.