The other guides are good but this one is a bit more foolproof, imo. Any which way it's time consuming though and you'll need a bit of the story unlocked and probably around level 20. 16 is the minimum. For this you'll be building up the Chimera. Also all directions are with the default camera. You can spin things around in game so be careful, it's hard to explain that way.
For starters here's a map guide that's very handy. I recommend consulting it with locations from below.
https://steamcommunity.com/sharedfiles/filedetails/?id=53033...Merchant Quarter. This is where you buy the Chimera from GebaN. Reaching there isn't the easiest to explain, but here goes. With default camera, continue upward. You should reach a merchant area (wooden crates) swarming with vampries. Above the center of this area should be a set of stairs between two buildings. A gargoyle should be on the stairs. Turn right as you go and you should be vertically above most of the map. In the corner of this little raised area is GebaN in the back. After killing him and his guards you can buy from him. The Chimera should cost around 100k so have your gold ready. If GebaN has any legendary (purple) Divine cards, you should buy them as well. He's guaranteed to sell this weapon although I'm not sure how often his loot respawns.
After doing so head back to the Castle. (Pause and Return to Hub). Go to the Transmutation station and select Chimera and press X to use it as the base ingredient. You should see four bars. The top is its Damage range and requires Legendary weapons to fill the bar. The second is it's Crit% and requires Legendary Destiny Cards to fill the bar. The bottom two are its Attributes. The top attribute (Damage increased when health is above) should be filled. The bottom (Gain max health on crit) requires Demon Powers to fill the bar.
For weapons, the guaranteed way is to find GebaN again and buy more. Again it's not cheap at around 100k a piece. I recommend selling all weapons that aren't purple unless you're using them and all Destiny cards that aren't Divine and/or Wicked that you don't want. (Edit: You can go to The Bottomless Pit and buy the Finger of Chaos from GebaN after level 38 or so. There's no bosses to fight as he's at the start. The Finger of Chaos is a decent weapon to upgrade as well.) The Cemetary Grounds is a bit easier in my opinion to get to GebaN. He sells the Pumpkin Hammer which isn't recommend for using as the base weapon as it needs Blink to fill and Blink is easily the hardest demon power to find/farm. To find GebaN in the Cemetary, keep on the left path Eventually you should fight a Gravedigger that throws skeletons at you. Just past him the ground switches to tile. From here stick to the left (avoid the first set of stairs to the left near the entrance as there is a somewhat lengthy fight with a few bosses.) Sticking to the left fight through a couple bosses and a lot of other enemies. Eventually you'll reach the back corner and he's on the corner to your left. You can buy the Pumpkin Hammer which you'll need 3 of at a time (or other Legendary weapons) to max that skill out. Again, buy any purple Destiny Cards if he has them as well.
For Destiny Cards when you're at the Castle the Patriarch will sometimes carry them. They're the purple ones and 20k a piece (unless they have a Divine/Wicked attribute at which point they can get far more costly. I'd avoid these unless you've got a lot of money saved). As mentioned you can find them from GebaN sometimes too.
Finally for Demon Powers you'll need to farm Sanguine Auras and Meteors. (2 Sanguines for one Meteor) per level. This is fairly easy. For Sanguine Auras go to The Royal Gardens. With default camera head forward and left past a bridge. You should reach a chessboard with a boss spider. As a note, he's guaranteed to have the power but not to drop it. The odds he drops it are fairly high though around 50/50. If he does or doesn't, you can pause and restart the map from there to fight him again.
For Meteor head to Sepulcher of the Damned. Default camera, head forward. There should be a Wraith on your left and skeletons on your right. After you kill them head toward the skeletons. A few more will spawn and at the end of this little hall a boss skeleton will spawn that has Meteor. Same with the other it's not guaranteed. You'll probably need at least 20 Sanguina Auras and 10 Meteors to start. What rarity (color) they are doesn't seem to matter so use the lower ones first. You may want to keep a hold of any Purple ones you find to equip or improve your outfit.
As you get those pieces use the Chimera as the Base by pressing (X) in the Transmuter and add the other parts three at a time with (A). Then press (Y) to transmute them filling up the bar little by little. It will take time and money to do so. Personally, I sold all weapons that were gold, green or plain to build up funds. You can transmute these three at a time for a chance at a better weapon but I didn't have much luck with the odds and it's less profit and a gamble. Also some weapons have a skill that makes them worth 3 or 4x as much. These are designed to sell as they can be work 10-12k.
EDIT: If you use the Finger of Chaos from Bottomless Pit as a base head to The Defiled Chapel to farm Purging Flames. Head left and clear out some fire essences and those skeletons that throw fireballs. Then head across the bridge and you'll find a boss Wraith that will carry the path. It's quick to get to him and some more of those fireball skeletons spawn. With him.
The following is some general notes.
Always make sure to use your Chimera as the base. If you've bought an extra to use it to upgrade yours make sure you don't get the two confused. (EDIT: It's probably better to get the Chimera and then use Finger of Chaos-es and/or Pumpkin Hammers to upgrade the weapon.)
If you happen to have a Power Stone that increases the damage or crit of a weapon, by all means use it on the Chimera. It will save you a lot of time and money buying weapons or destiny cards.
You can use other Legendary weapons you find but it's random and rarer to find them. I personally used Lady Luck, but it's random finding that weapon. Plus some Demon Powers are hard to farm especially Blink but also Dark Mist, Berserk and Pull. Sunray, Shockwave and Purging Flame are in the middle of difficulty in farming. If you have a Legendary weapon you want to build up fast it should have either Sanguine Aura or Meteor as the skill and use the guide above. The other viable options for farming are Boomerang (Royal Gardens to the right in the maze), Time Loop (Befouled Tomb, first left and up the stairs) and Diamond (Watchtower Ruins, first big room you enter).
EDIT: You can transmute three demon powers in the bottom for one random demon power which can help find powers like Blink. Being random it can be time consuming though.
I also don't really recommend using your found Legendary weapons unless you have a duplicate. A few are needed for other achievements (Chicken Cannon, Power Foil)
You can apparently also use a rare (gold weapon). It may take less time as you can find the Cards and Weapons at that rarity faster to build those up. However you'll need to add a third power to the weapon with a Power Stone. And again with the difficulty of finding the right Power Stone, it can take some time.
EDIT: Treasure Hunts and Bounties are the best way to find Power Stones. If you go online you can redeem key under options (all accessed from the menu you get when you press the ≡ button.) Also you can apparently switch Power Stones out and keep your transmute progress if you don't like your third. I ended up maxing out the power of a third power stone and then switched it to find more items.
If you're not at or near level 50, you get a few Purple Destiny Cards as you level up as well as some Demon Powers. Also you can keep your Gift Boxes and Transmute them until you have a Legendary one for a good chance of a weapon or card. You can do this after reaching Level 50 (on the base game, a DLC adds to level 60.)