The Witch's Cauldron sells a variety of items, and the contents of her shop changes naturally every hour, or you can refresh it with 100 diamonds.
Purchasing from the Witch's Cauldron is a late-game consideration due to the high diamond costs. Early on, just invest in Summoning.
She sells:
This article discusses each of these items, and whether or not they are worth buying.
This is the most consistent place where you can summon specific non-limited dragons you want. The Witch shop sells relationship points which you can collect to summon the dragon you want.
A Heroic dragon requires 80 relationship points to be summoned.
A Legendary dragon requires 160 relationship points.
So without taking into account possible discounts, it will cost 31,800 diamonds to summon a Legendary dragon and 3976 diamonds to summon a Heroic. You can also buy them with Ancient Coins.
It may sound like a lot of diamonds for a Legendary dragon, but with buying relationship points, you can choose exactly what dragon to buy.
By contrast, if you spend 30,000 diamonds in Advanced Summon, you have a 67% chance of obtaining 1 or more 5-star dragons, but even if you hit that probability multiple times, there is no guarantee that it will be the dragon you want.
Thus, the Witch Shop is the easiest and most reliable way of getting a particular dragon you covet.
Skill Slimes are used to increase the skill level of your dragons. The Witch's Cauldron can sell:
It is a very good idea to buy Heroic and Legendary Skill Slimes, especially on discount. You may wish to mainly wait for them to be on discount before buying one, but if you are in need of Skill Slimes, I happily encourage you to buy them whenever they appear.
I do not think it is necessary to buy Rare Skill Slimes, as it is so easy to fully skill Rare dragons with duplicates from Summoning, but it is up to you to decide if it is worth it to you.
The Witch's Cauldron can also sell Super Slimes of 3☆ to 5☆ rarity.
I do not think they are worth the purchase, as it is so easy to get fodder and level them up. However, if you only need just a little bit more to 6-star a coveted dragon, then absolutely you may purchase 1 or 2.
The Witch's Cauldron can sell:
These are very worth the diamond cost! It can be difficult to come across relationship points for enhancing your dragon - especially as you will need so many to enhance even one dragon to +6. Note that they are very expensive, but enhancement points are difficult to come by. Buy them whenever they appear on discount - or even without if you are desperately in need of them.
It is important to note that the cost of enhancement points is the same as relationship points. If you are looking to enhance a dragon that is not a Topaz or Limited dragon, you may wish to also pick up that dragon when it appears in the Witch's Cauldron as relationship points, especially if they are on discount.
Again, I do not recommend buying Rare enhancement points, as it is so easy to get duplicate Rare dragons.
The Witch's Cauldron occasionally sells Wings and gold - you can usually ignore these, but sometimes they are up on discount.
For Wings, absolutely you should buy them on discount as you will always need Wings, and it is great to get them cheaper whenever possible.
For gold, I usually do not recommend buying them with diamonds, but if they are on discount, they may be worth it. You should decide if it is worth it to you - if there are other things which you need to spend your diamonds on soon, you might wish to abstain. Late game when gold becomes one of the largest blocks to progress, you may wish to pick it up whenever it appears on discount.
I grouped these together as you generally don't want to buy these items.
The runes are very expensive, and barring a Legendary 6-star rune with all the right substats at 4%, you should refrain from buying them.
It is pointless to buy the other items as they are very easy to farm. The only time you may wish to buy them is Evolution Material when you have a dragon who is very nearly built to completion, and it is not the correct day to grind dungeons for their element.