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The easiest way I've found is Cave of Ordeals 3. Simple as that.

Majin:
Level your characters until they can kill off the creatures and then start walloping on the Galactic Demons. Yes, even scouts and thieves can take them out easily. The last two gun attacks are long-range so you can steer well clear of the GDs and shoot to your leisure. The Fire attribute on the attacks may cause you to do less damage to some, but your gun's normal attack should still reach those from safety.

With the Brawler and Warrior, fists and swords respectively unless you want to teach the brawler to use a sword. The Ninja, either fists or swords. Winged Slayer is good for killing any and/or all of them especially if you surround the block with a Ninja (fists) in back, Warrior (sword) on one side, and Brawler (sword or fists) on the other. Big Bang with the Ninja since you won't have room for Winged Slayer and a Ninja can reach the back with only 1 or 2 more movement increased from its base. Then use whichever other you wish to do further damage and the final character to mop up.

If your characters are too weak for now, try my favorite leveling spot for solo characters. The dragon in the woods. (I believe it's where the 'impossible' fight takes place with the alternate overlord(s)) It starts at lvl 40 and you can up it as needed from there. My Ninja got to 200 there before I took him to CoO3. Ninja are easy to level because of their dodging ability so just go for it.

If you still can't kill the Galactic Demons after leveling on the Dragon, start fighting the catgirls in CoO2. They are also a good place to level up, just a slight bit riskier. And if all else fails, pass the Bonus Gauge Boost in the Dark Assembly and take on CoO3 with whoever. Then when you see EXP bonuses in the list, pull out the characters you need to level and run them as far from the GDs as you can. They'll level slowly, but they will level.

Angels:
These are much easier than Majin since you need two less classes at half the level. The hard part is the Clerics since they're healers and not so good with Melee. However, even they are easy.

With your cleric, have her create a Galaxy Mage. Then level the mage up using XP bonuses in CoO3 until it learns Omega level magic. Teach Giga and Omega level magic of all 3 elements to your cleric. (casting on a galactic demon of your own is a good way due to their high Res stat.) Now, have her take on the dragon in the woods. It's lvl 40. If you pass the Stronger Enemies bill a few times she'll level to 100 in no time.

With your Knight it's even easier since she's made for damage. Simply go in and wallop on the dragon until she's lvl 100. If you need to level her up so she can take on a lvl 40 dragon, you need better equipment. Go item world to level your people up while prepared to steal higher quality equipment. (high level thief is nice here) Or, level up in the item world while hunting specialists like Statisticians or Armsmasters. (the second being extremely important for getting your thief's gun skill up so it can take care of itself in CoO3)

The Archer, use the bow skill that paralyzes the dragon and start pelting it from out of range of its skills.

If all else fails, use team attacks on the dragon to level multiple characters at once.

EDF Soldiers:
Armsmasters Armsmasters Armsmasters.

And a Scout.

'nuff said.

(ok, a little more)

You get a lot of low-power free items for fighting the dragon. Sell anything that doesn't have a Statistician or Armsmaster in it and dive into those that do. Put as many of both on your gun as possible without going too far over the cap. (300 for Statisticians and 1500 for Armsmasters) Now just go shoot shoot shoot. If you want a level where you can just shoot to your heart's content, either stick around on a level with Invincibility and a Clone panel in the item world, or the level with the catgirls and succubi in Jotunheim. Invincibility for both or invincibility with unlimited enemies. That's the key. Lvl 30 Guns on your Scout will not only allow him to unlock the EDF Soldier but will permit him insane damage in CoO 3 for his level.

That's all I have to say. . . hope it wasn't hard to follow, I know sometimes my explanations are.

What level are your thieves?

Also, what's your best gun? You literally want to equip one thief with your best gun, and make sure it has as many Armsmasters and Statisticians as possible on it.

If you need to level thieves without risk it'll take a while, but put them in the Jotunheim level with the invincibility field. You can have your thief kill the catgirls and succubi one at a time or as one powerful creature without risk. If you want to level up your guns I recommend using one thief and a low-level mage. Your second shouldn't have any counterattacks so it can't accidentally kill the enemy.

I recommend for faster level gaining you toss two catgirls together and then each other creature onto that creature. Tossing a catgirl onto a succubus of equal level makes a succubus so you have to throw 2 catgirls together first. Then they can't use Hip Attack to heal themselves. If you want to level up your guns then make a high-level succubus so you get more shots before it dies.

Once you have a Fire element special attack with your gun you can use the first attack on dragons weak to ice and the fire attack on those weak to fire. For the ones weak to wind, just use whatever. When your Thief is strong enough to kill a dragon once, use the Triple Exp proposal and kill a dragon. Then pass it again and kill it again. It's a very fast way to level since it's one strong enemy in a map. You only have to worry about getting hit by one creature and you don't have to do combining like in Jotunheim.

As I said, once you have the final gun skills, you can kill the Galactic Demons from out of their move range. (hopefully, by the time your thief knows these, s/he'll be strong enough to keep the GDs from using their special attacks.) If nothing else you can kill the center demon in the front row since the Base Panel blocks it from advancing and have someone else mop up.

In one:
Go to Celestia level 1 with the 'Maderas's' and keep team attacking with the weakest people you have. When all your levels surpass them pass a bill to increase enemies levels.