Hand of Fate

As well-detailed as Dragoner28's guide is, I'm pretty sure this solution will allow you to easily defeat the Dealer with virtually no effort whatsoever.

Just progress through the levels like you normally would, but when you get into the three-block stretch right before the boss, make sure of two things. One, make sure you are afflicted with Midas Appetite, a curse that causes you to eat gold instead of food. Second, make sure you either found or purchased the Emergency Ring, which gives you a free resurrection at the cost of all your food. If you're missing either of these, just forfeit and try again until you have both.

I'm not sure if it's a glitch or what, but as long as you have both of these things in effect, you become invincible during actual combat. In my case, I reached the Dealer with 56 gold left over, and every time my health depleted, it just reset back to 56 health infinitely. I actually went back and re-tested this; the second time I got there I just left my controller sitting there, came back half an hour later, and the fight was still in progress.

Hopefully, this alternative method will work for anybody who's having trouble beating the Dealer legitimately.

To beat this Encounter you have to survive 4 Waves
First Wave: The 3 Dust bosses
Second Wave: The 3 Skeleton bosses
Third Wave: The 3 Pest bosses
Fourth Wave: The 3 Lizardmen bosses

Suggested Equipment - thanks for Input @ CounterInsurgnt
Weapon: Giant War Hammer
Unlock: item is given to you upon completing Holy Forge.
Value: $50
Damage: 40
Abilities: Holy, Finishers
As you string together consecutive attacks you deal greatly increased damage. Press to trigger a force explosion.
Alternatives:
Huge Hammer
This weapon has an AoE stun spell which make the boss fights much more easy. If you use it clever, you can stun all bosses at once. The AoE spell is also doing damage to all affected enemies in range. Another advantage is, that the AoE spell is interrupting blockable attacks.

If you already have unlocked Dragon Tongue, it is advised to use this sword, as it is the best weapon in the game:
Dragon Tongue
Unlock: Complete Petward the Monk.
Value: $60
Damage: 40
Abilities: Fire, Finishers
Each hit ignites the target. Press to shoot fire from this sword.

Artefact: Lightning Crash
Unlock: Complete Minotaur's Maze
During combat, press to knockback and stun all enemies with a crash of lightning.
This artefact also stuns all bosses. But for a longer timeframe than your hammer. You should use it at the third and fourth wave. As the first two waves are very easy. Also dust and skeletons do fewer damage than lizardmen and pest.

Armor: Immovable Object
Unlock: Defeat the Queen of Scales
Value: $35
Defence: Heavy
During combat, this armour grows stronger with every blow sustained.
As you have to fight 12 bosses, you will be hit often. In my opinion this armor can absorb the most damage of all available armors, as the "growing stronger" effect is very useful for a long fight.

Helmlet: Hernes Antlers
Unlock: Defeat the Jack of Scales
Value: $20
Lizardmen take double damage from the player's attacks.
In the last wave, the bosses can´t die before the Lizard King is dead. Because of that I use this helm, to kill the Lizard King with a quick stun -> weapon combo -> stun -> weapon combo cycle

Useful Rings
Ring of Justice
Unlock:
Using a weapon ability causes all enemies to be stunned for 10 seconds and for the ring to be destroyed at the end of that combat.

Occult Ring
Unlock: Complete Cursed Treasure
The player inflicts 2 more damage for every Curse they have.

Veteran's Ring
Unlock: Complete Blood Auction VI
After counter-attacking the same enemy 3 times, that enemy is marked and receives double damage from the player on all future hits.


Weapon and Artifact are very important for my tactic:

1. First Wave
The bosses always spawn in front of the dealer. During the spawn phase, run to the middle boss and press . With this your hammer, will do an AoE stun. Now you have time to attack the boss with which you have the most trouble. The dust bosses are very easy. My tactic was to kill the dust king at last, because he is more a threat to his own allys than to you. His range spell is doing damage to his own allys and if he hit them with his bombs, you have time to land a few strikes during the stun.

2. Second Wave
During the spawn phase, run again to the middle boss and press . My first target was the skeleton column because, the column is spawning new enemys.
General tactic:
- Keep the bosses near together and try to kill them at once
- If one of the bosses is dead, the other bosses trying to revive him
- While casting the revive spell, the bosses lit green. Try to kill them from behind during the cast
- The hammer is doing bonus damage to skeleton. As the Range boss has the smallest amount of life of these bosses, you need only a few hits for this boss. Try to kill this boss at last
- The Skeleton King can spawn suicide bombers. If the Skeleton King is spawning them stay near the suicider to trigger their suicide bombing. You have around 2 seconds to run away if the suicide is triggered. The suicide is also doing damage to the other skeletons. So use it to your advantage

3. Third Wave
In my opinion the most difficult wave. Your first target should be the range boss. Because the other two bosses are very easy to control without range enemys.
The Pest King is no danger, as he is running slowly and after a while is doing a three hit combo. After the three hit combo he is for a few seconds vulnerable and is doing nothing.
The third boss can teleport and is the most difficult boss of this encounter. To avoid being hit, always move. Don´t stand still. You can only attack if this boss is stunned or is doing nothing. After his teleports he is standing still for a few seconds. But always keep an eye on him, as his teleport moves are doing a huge amount of damage

General Tactic.
- During the spawn do as always the weapon spell. Focus the range boss afterwards
- Use the lightning artefact to stun all bosses. If you use it 3 times and focus each time the range boss, she should be dead.
- Be always on the go and kill the teleport boss as next
- Only attack him if he has missed you with his teleport attacks, because he is vulnerable after that
- The pest king is slow. If he is coming near, you can easily do damage with the weapon spell to him, because this spell is doing Area damage to a zone which is greater than his claw range

4. Fourth Wave
Very easy wave, if you have the Hernes Antlers helmlet. Keep in mind that while the Lizard King is alive, the other bosses can´t die. But they can be damaged.

General tactic:
- Use the weapon spell and the artefact to do a stun -> weapon combo -> stun -> weapon combo cycle. Fokus the Lizard King at first. If you have the helmlet, the Lizard King should be dead after the third weapon combo
- Kill the range boss afterwards. As the weapon spell and artefact has done damage previously to this boss, you should only need a few hits.
- The third boss is very slow. Mostly he is trying to hit you with his tail. This move has a very long loading time before he is doing the hit. You can simply roll to the side and hit him afterwards.

Additional Tips
If you struggle with the curses this tip from LunarKnightX will help.
One thing that I try to do that I find helpful is the "Nymphs of the Sweewoods" event card, if you run into it early enough in the game anyway. It resets you to the state you were in when you fist started the game (Back in the Jack of Dust tutorial, without all the bonus gear from the items you got from the dealer.), then gives you a shield and blessing that lets you draw a gain card every 5th move. This unfortunately means you'll lose ALL your gear when you use it (hence hoping it appears early enough to re-equip yourself.). The big draw here is the fact that it also deletes your curses, ALL of them, even the ones associated with the boss you're playing against, so in the dealer game you'll stop getting new curses every floor and actually be able to get yourself some blessings before you hit the big boss rush.

Alternative to the above mentioned stunning tactic: Lil Miss Cherry has hinted that there is a heal artifact, which can be used 3 times
Time Heals
Unlock: Complete Goblins
Value: $25
During combat, press to twist time and revert all damage taken in the last 10 seconds.

APPRENTICE DIFFICULTY DOES NOT WORK.

Time Heals Artifact It i s pretty much essential, it gives you 3 lives also If you have Dragon Armour that massively helps but not as essential as Time Heals.

DO NOT LET THE COMPUTER CHOOSE YOUR DECK, Greatest error you can make

Customize your encounter deck. - At first I had as many cards such as Metal Ore, field of poppies, local peasant etc where I could quickly get to the dealer, this is a waste. You need to gain as much max health, gold, food and equipment as possible.

Make Sure The Maiden, Soul Gem, Ghost of The Sea, Lonely Bard, dead-mans gorge are equipped. I managed to get my health to 145

Next Make Sure cards which give supplies, gold and food are included even ones where mild combat is required so. My deck included: Mister Lionel, Cave Rescue, loan, tavern wager, a wife's fury, the altar, wandering minstrels, food wagon, heros remains, embertown hero, angry guild master, trading post, cave shelter, twisted canyon

These are all cards which have great rewards for minimal effort.

Now customise your equipment, include only your best weapons, armour, shields. Only have two rings in your deck. If you fill up your deck with rings then the shops and gain cards will be dominated by rings, we need weapons, Armour and artifacts. Make sure one of those rings selected is the Emergency Ring. Now include the Lovers Card, demand payment, and you are guaranteed to get those two rings. If you include the cursed treasure card you have a strong chance of getting Midas Appetite if you want to take advantage of the cheat.

If you have unlocked TIME HEALS ARTIFACT, make sure that is the only artifact in your equipment deck, which increases the chances of it being drawn or in a store. This artifact essentially gives you three lives.

Visit Every Shop, sell your surplus supplies and make sure you buy the best equipment you can

The most difficult wave is the Lizard Kings Wave - Hermes Antlers deals double damage so they are helpful. However until you kill the Lizard King none of the other enemies will die. I learned this after 15 minutes.

If you start the battle and are happy with your inventory, you have raised your health, and you get far but end up dying, make sure that you dashboard out, reload the game and you can start the battle again.

Got somewhat lucky after a year of owning the game.
.
I had:

Dragon Armour
Damocles
King's Ring
Emergency Ring
Veteran's Ring
The Warhammer you get from Holy forge

That combination gave me multiple 'lives' to get it done. It was a custom deck.

Edit: Also you cannot get this achievement by beating The Dealer on Apprentice difficulty.

I agree with all of the above but wanted to add that on xbox one, you can hit home immediately after you die in this fight, then quit the game and restart. When the game reloads you will be one card before the dealer again so if you have the perfect set of equipment you dn't have to spend another 35-45 minutes getting back to this point, just refight the dealer.

Also works with bosses like the kraken etc.

Pretty simple really. First off you need to go into this encounter with as much health as possible. Then the only must items are the Blood Crescent sword and Time Heals. All you need to do is use the swords ability on any of the bosses you have trouble defeating normally. For me I used it on the King of Skulls (he reanimates and calls forth other skeletons), the King of Plague (he will perform a few moves I found to be irritating) and the King of Scales (you have to eliminate him before anything else can be defeated). If you get hit too many times or you use the swords ability more than I did and your health gets low use the Time Heals. The Time Heals are useless if you start this encounter with low health which is why you must start as healthy as possible. I also put the Kraken encounter in my deck in order to get the helmet it drops. The helmet has a chance to instantly kill ANY enemy (bosses included) that does any kind of damage to you. Other useful items are the Skeleton King Shield for bashing skeletons, Armour of Cold for slowing nearby enemies and Hag's Wraps because they are awesome. Also any rings you feel would be helpful.