There are a few ingredients you'll need to break the game wide open. Before you start, unlock as many Heirloom slots as you can, and craft a Magic Smoke Bomb if you can. The last step to becoming overpowered is generating unlimited Heirlooms, so having all three slots unlocked before you start means you only have to do it the hard way once. If you can't craft Magic Smoke Bomb, Book of Spells would be my second choice, and a number of other items could potentially be useful, including: Strange Egg, Suds, Punchbag, Holy Water, Capitalism, Comforter, and Merchant Ring. I would recommend the Wizard mastery, so you can reset after your first shop if it doesn't sell MSB (Don't forget to gather money before visiting your shop, MSB isn't a cheap item). Also, I don't know if Scribes spawn outside of Hard mode, and you will need a couple Scribes once your build is finished.

Step one is being able to replay stages as many times as you want. After the first few levels of a quest, the levels are large enough that Magic Smoke Bomb (MSB) on it's own is enough for this. Play through the level enough to charge the MSB, then collect as much loot as you can (playing more to uncover more of the stage if there's a lot left covered), and get out. Smoke Bomb, Key of Undoing, and Infinity Key are also helpful for this, but they're single use, so they are really only useful for making things a bit more efficient. Once you have infinite Shops and infinite mana, Punchbag is a great source of infinite stages.

Step two is finding a way to profit from your infinite levels. Every level has at least one chest with a random chance for more than one. If your inventory isn't full, opening chests is dangerous - you might end up with an item or Omen that kills your run. At first it may be necessary to take that risk, but try to fill your inventory with consumables as quickly as possible so that items in chests drop on the ground and you can decide if you want them before adding them to your inventory. Minions and Bones work best for this, and it's good to have at least one armor item once money isn't an issue because you can use it in shops. As long as your inventory is full and you're not in danger of dying from a mistake or two, you will eventually find useful things in the chests. This process starts out very slow, but speeds up as you become more powerful. Also pay attention to Strangers. Nomads will sometimes sell things worth buying or buy your things, and Hungry Plants can be used to get rid of dangerous items in a pinch. Be careful using Hungry Plants without Mucilage however, as they might take an important item instead. I wouldn't recommend using them unless you're desperate, or early enough in a run that restarting wouldn't hurt that much.

Step three is getting infinite shops and a Merchant Ring so you can sell unwanted items. Shop Key or Capitalism will give you unlimited access to Shops. Keep in mind that Tablets and all Omens except Dead Rodent can't be sold, and that Tablets especially are difficult to get rid of. Holy Water directly removes Omens, and Magic Dark Ink replaces Omens with other Omens, which moves them to the end of your inventory. Mucilage then allows you to feed them to Hungry Plants without risking the important stuff. You will never accidentally end up with a tablet, so as long as you don't pick one up, they won't become an issue. The only reliable way I have found to get rid of Tablets is with Blind Justice, and that requires getting rid of everything above them in your inventory as well.

Step four is making everything faster and more efficient. Get the number of monsters per stage down to one and not only is the stage solved as soon as you start, but the entire stage is empty cells suitable for loot to spawn in, greatly increasing your loot per level while also making levels faster to complete. Mjolnir and Ouroboros are good for this, and Lyre can also work if you have a strong source of mana. Book of Spells or Magic Amulet is also a must-have before you remove all the monsters. The mana generated by a given cell when it is revealed is the same as the number in the cell, so as you reduce the number of monsters in a stage, you also reduce the mana you get from completing the stage. Magic Amulet is dropped by some monsters, and its effect stacks, so if you have a way to kill monsters, this is an easy way to get unlimited mana.

Step five is setting up your inventory with whatever combo you want and becoming invincible. Keep the order of your items in mind, you'll eventually want to find a Scribe to copy your first three items into your Heirloom slots and another Scribe to duplicate them so you'll never run out. You can only use one Heirloom per quest, so choose items that help finish putting together a broken run after a promising start. Magic Smoke Bomb is an obvious choice, and Paragon Rod and Lance are also good choices. I haven't done enough testing to know which monsters, but some monsters can drop Magic Amulet when killed, so Lance can be an indirect source of mana.

As for what to target for your final inventory, there are several good options based on your goals. To fill your Codex, Look for a Paragon Rod and items that increase the drop rate of loot (listed in the next section, as they are also used in the Token farming build). Combine this with Mucilage to dispose of the items after you pick them up, and you should be able to complete you Codex fairly quickly. For an endless run in Endless Multiverse, use Magic Hammer to strengthen your armor of choice up to around a million durability; find a nomad selling God Potions or Halos, and buy a few dozen from him; Accumulate a few dozen souls from any of the numerous items that give you those; Activate a few Hearts of Steel. You can use Alcohol to increase your max life faster than anything else if you use a Canteen to remove the status effect afterwards; keep in mind that this will also remove the Heart of Steel effect, so save those for after if you choose to use Alcohol. Finally, use Suds or Bubble Wand to store extra items in bubbles, to effectively make your inventory larger.

By now you've acquired Merchant Ring to sell unwanted items, Shop Key or Capitalism for unlimited access to shops, Mjolnir or Ouroboros to remove all but one monster from the stage you're attacking, and maybe Purify to remove any Stage Mods you come across. You can attain any combination of items you want by entering shops again and again until they sell what you're looking for, so what should you be looking for?

The best combo for generating Tokens is Chisel, Book of Spells, Event Horizon, Astral Spike, and Ethermancy. You need Chisel, Book of Spells, and either Event Horizon or Astral Spike to get started, and all of them to be at full strength. In addition to this, anything that increases the amount of loot that spawns is also helpful. Fancy Ring, Golden Amulet, Life Amulet, Diamond Amulet, and Horseshoe are good for this. Glowspore is also nice to have if you're using any magic items other than Chisel. If you come across Magic Hammer, pick it up. Doubling the durability of your armor every cast gets out of hand fast, and even armor that starts at 2 durability will have over a million durability after just twenty casts. Combined with Bandana, this lets you use shops effectively for free, much faster than collecting money in between shop visits. I haven't tested it with Capitalism yet, but since you're not spending coins I don't think shops will remain open, so make sure you have a Shop Key before you equip a Bandana. You'll also want Shopping Bag for this combo. If you have to leave and come back every time they don't have what you want, you're not really saving much time over just collecting money. With Shopping Bag, you can just buy whatever they're selling to make them restock, then sell it right back to them.

Chisel converts each type of loot to the next higher type. The chain of conversions is:
Coins -> Diamonds -> Hearts -> Chests -> Tokens -> Tokens and Artifacts -> Tokens
It's important to note that Tokens are converted to Tokens, not ignored. This is because every time something is converted to a token, there's a chance it will be converted to a Mega Token instead, and Mega Tokens will be converted back to normal tokens. So as you convert all the loot in a stage, don't collect the Tokens until the end, but collect Mega Tokens as you go.

The larger the stage you choose to farm for Tokens, the more efficient you will be. I like stages around size 500-600, because any larger than that and it becomes tedious to look around the whole stage for Mega Tokens every time you cast Chisel, but larger also works. Cast chisel anywhere (I like to start in a corner). Event Horizon and Astral Spike will cast it on the entire row and column, and Ethermancy will apply the Ethereal Aura to all of the cells. Collect a heart, and Book of Spells will recharge Chisel. Chisel again in a row and column that aren't Ethereal yet, and in addition to that row and column, all rows and columns that have been targeted previously will be upgraded again. Keep going until every cell is Ethereal except a few in a clump somewhere. Now that everything is Ethereal, everything will be targeted every time you cast Chisel, so try to plan out the last few cells to get as many hearts as possible. Every heart is another cast of Chisel, and every cast of Chisel is a couple more Mega Tokens, so try to draw a few extra hearts of of those last few tiles. Every time mega tokens spawn, your screen will be drawn to one of them, so if you screen stays put after a cast, you didn't get any Megas and you can cast again; however, if there's one Mega (or Purple Diamond or Dark Chest), there may be more - your screen only gets kidnapped once, no matter how much high profile loot there is. Look around and find the rest. Every time you cast Chisel, the Megas are converted back to normal Tokens, so pick them all up before you cast again.

Using this method, you can expect 2000-3000 tokens per hour, more than half of which come from Megas. Everything worth buying in the shop can be bought for a total of around 1000 Tokens, so unless you want ALL the Masteries and ALL the cosmetics, half an hour of farming should be plenty once you're set up. It takes a pretty long time to set this up though, so maybe do an hour to be safe.

The Stage Mod Burning states that monsters are never in burning cells. I have found this to be true in Elemental stages as well, and I suspect it's true in general.

Tunnel stages are smaller than usual stages, but each subsequent level is larger than the last. This increased size remains if you complete the stage and then re-enter it with a Key of Undoing (I haven't checked using MSB, but I assume that the size increases as soon as you click the staircase, and doesn't go back down), so these stages can get unreasonably large pretty quickly. After completing a 3 level Tunnel stage twice, it was up to size 2000. There is a considerable amount of lag on stages this large, I would not recommend letting them reach this size.

Drops I've encountered. I went around and killed a bunch of strangers, but not enough to find all the drops. I'm sure Blacksmiths can drop more, I only killed a few Scribes and Bagmen, and I'd be surprised if Waterboys never drop Suds. Everyone can drop Bones, so I didn't bother listing that.
Nomad can drop Merchant Ring, Shopping Bag, Wallet, Big Wallet, Golden Amulet, Bindle.
Blacksmith can drop Bronze Helmet, Steel Helmet.
Waterboy can drop Bubble Rod.
Strange Plant can drop Miracle Seed, Demon Seed, Sapling, Mucilage.
Mercenary can drop Whip, Banner, Brotherhood Crest.
Scribe can drop Voucher.
Bagman can drop Totem.