Heya,
If you're reading this than you are probably in NEW GAME ++ , if not, then i highly recommend it since you will get extra spellslots and thus will be able to make spells beyond your wildest dreams.
The spell that I am about to show you will be making use of the extra spellslots of NG++. As the guide goes on I'll try and have NG / NG+ additions as well, from here on you will be able to add 1 material at a time and with this have the ultimate spell in the end

Lets start with the basics, as mentioned earlier this is what you would be using when just starting a game. Do note that for the purpose of this guide I will only examplify A rank materials.

The base
- Chaos Butterfly (Fox + Fire)
- Crystal Ball (Banana + Atom)
- Extract of Monday (Flower + Water)

Extra materials* (in this order for every NG cicle)
- Spider Silk (Fox + Fan)
- Rock (Fox + Leaf)
- Seashell (Tree + Fire)
- Vulcan Wrench (Grapes + Water)
*= The extra material slots can be found in the Jungle Temple level for the first time in every NG cicle.

When testing this on the single stone dummy I counted that he had died over 20 times with a single cast whilst on the highest possible health which is 100.000 HP. Afterwards he had around 50% health left, so lets say 50.000 HP. Do note that this amount of damage was dealth while using a 75% damage buff while stand ing still by using (Tonna Bricks). Because he had already died several times while trying to see the DPS we will have to use math in order to find how much one cast does.

Quite easy really: (20x100.000)+50.000= 2.050.000
To remove the 75% buff: 2.050.000/1.75= 1.171.428

So one cast, over 2 million damage. This is the strongest spell as of my knowledge. From what I've read some people have a mob killer and a boss killer spell. But with this spell you will be able to clear your entire screen so it leaves not a single enemy behind, whilst also being able to kill the boss in an instant

To put this all in prespective, the final boss* (Azazel) of the game in NG++ has a total of 10.000 HP and 5.000 HP in the second "stage"

*= Yes, I had to go back and check her health for this

Due to a projectile count which might just be over 100 (the count can be inconsitent due to the Spider Silk), each with their own particle effects this spell could be very taxing for your computer. I highly recomend to either turn off as many extra effects as possible.

Now I bet that there are many of you out there who think they can outperform this spell, and all I can say is; please do, the more crazy OP spells we get added to our arsenal the better!

Thank you for reading my guide,

|n cese tath I mad ani tipoz, ples tel m ni teh commemts belo