When you land Big Sunflower - bnsgold.co

When you land Big Sunflower, you get one stack of a buff called Photosynthesis. It lasts for five seconds. If you manage to get five stacks, then you will get a special buff called Saturation. With Saturation, you can instant-cast Sunflower for zero MP for 3 seconds, which becomes five seconds when you get BNS items the top-tier secret skill book for Sunflower.

So the idea of the build is to take advantage of this effect. You need to set it up with the right status effects, maintain them as you cast Sunflower to get Big Sunflower, then slip in some MP regeneration to keep the whole thing going until you get Saturation. Here is how the skills work. For the first skill BNS items, Rose, max out the first form. That gives you faster cast and projectile speed and an extra point of MP regeneration, which you need. Next, take the 4th form of the Sunflower skill- the others give you Hornet, but Sunflower and Big Sunflower are keys to this combo.