Yes, there are DLC for EXP and Money, but let me just say that there is absolutely no need to buy them. Because it is pretty easy to grind.
However, yes, these farming methods are for end-game. If you want to overpower yourself through DLC early on and, IMO, ruin the experience for your first playthrough, then DLC is the answer. And yes, DLC is still faster than this farming method, but it isn't that painful to just grind in-game.
To put it simply...
Also known as "Multiplying ATM" (based off of ATM machines)
The reason for this is because of his unique characteristic as a FOE.
He creates a clone of himself, which is nothing different from if you fight the real him. Therefore, no matter how many times you kill his clone, he creates a new one in four steps. So you never have to leave the labyrinth and can just fight him until you run out of TP.
Now then, what do you need to farm him efficiently...
2 Necromancers (1 with Zombie Powder, 1with Merciless Shield)
1 Reaper (with max level Soul Gather)
The item that when equipped, sends 10% of the EXP you get in battle to all members of the Guild not in your party.
Reaper's Soul Gather doubles the EXP you gain while active. At max level, you gain double experience.
Necromancer's Zombie Powder is the key to this farming. For you see, poor, poor Multiplying ATM has normal resistance to instant death. Not "not very effective" like most FOE but "normal" Although it depends on your LUC, you can usually kill him by the 5th try.
The other Necromancer's Merciless Shield is simply to prevent anyone from receiving damage. Before you take out the first ATM, you will have to rely on the 3 Ghosts you can summon normally as a shield, but once you Zombie-fy one ATM, you get a 9999HP shield that will most certainly last until you create another one.
This leaves 2 open spaces where you can put anyone.
Reaper, 2 open spaces can do anything they want.
Necromancer 1 uses Merciless Shield
Necromancer 2 uses Zombie Powder.
In Advanced difficulty, ATM gives your party 170000 EXP. With Soul Gather, that makes it 340000.
Divided by 5, your 2 "to-train" characters get 68000. All of your non-party Guild members get 34000.
In total (assuming you have filled your Guild), that is 68000 + (34000 x 25) = 918000 EXP.
And it usually only takes 2 or 3 turns to get this.
I was able to reach 99, retire, and get back to 99 for ALL of my Guild characters in two days.
On top of that, if you defeat ATM within 5 turns, you get his special drop. Along with his normal drop, you get 18800en worth to sell.
By the time I finished grinding my levels, I had over 10 MILLION en. More than enough to buy and equip my entire Guild with the weapons and armor I wanted.
So yeah, 2 days is all it takes to get all of the EXP and En you will ever need. And all you have to do is spam Zombie Powder and Merciless Shield, walk 4 steps to respawn a clone ATM, fight and repeat.
In this game, you need Ingot to forge and power up your weapons. You can also use the original material to Forge, but when the material is FOE/Boss stuff, it is much easier to just use Ingot.
What I consider the most efficient way to farm for Ingot is by fighting this guy...
This skeleton is a Straum 3 FOE. Just like our friend, ATM, he will respawn himself if you take four steps. By end-game, you should be strong enough to take him out in 2 turns on Auto.
So fight him, Auto and let your party slap him to death in a couple of turns, walk four steps, repeat.
It takes 6 of his material to get the weapon you can create with his material to +5. By melting it, you get four Silver shards. Melting 5 of his +5 weapons get you 2 Silver Ingot, which is enough to make any weapon +5.
While there are other ways to farm Ingot, this is the easiest way because of how few control inputs you have to do. The entire battle is done on Auto. You can easily watch TV while doing it and rarely have to bother to focus on the game.
Also, the weapon you can create from ATM gets you Gold shards. So after farming him for EXP and en, you can use his now 99 material stock to create some weapons to melt for Gold shards. However, compared to the method for Silver Ingot above, it costs much more money. Also, the weapon his material creates happens to be the 2nd strongest Katana (1st strongest is the HIDDEN BOSS' material) that can also be considered one of the best Katana to use. Don't use up so much material that when you go to create the Katana to use, you don't have enough material anymore.