Just pasting calc's from this guy on the wiki.
Method 1:
Well, as we see here, the "Floating Island" is practically nothing near the explosion created by Yuri Zahard, with his technique called Rose Shower:
http://unionmangas.net/leitor/mangas/Tower%20of%20God%20-%20Season%202/225/53.jpg
I am going to use the measurement of 185m for the length of this Island, because it looks more like a stadium, given that it has an arena of battles inside:
https://en.wikipedia.org/wiki/Est%C3%A1dio
And as we see in Pixel Scaling, the explosion reached almost 3km in height:
Now, let's calculate the DC:
As we see, the explosion is a cone, so:
Volume = (pi x r² xh) / 3 Volume = 6.594.574.306 m³ / 6.594.574.306.000.000cm³
I will use the amount of spray, because as it is seen in the image, the roses have been reduced to a very small fog:
Spraying = 214.35 J / cc
6,594,574,306,000,000 x 214.35 = 1.4135470024911e18 J
337.845842 Megatons = Mountain level.
Method 2:
As the friend said that this explosion covered ALL THE AREA, we see in Pixel Scaling, the whole explosion covered an area of 5680m (measurable Area):
Now, let's calculate the DC:
The image does not make clear the area that was affected, but based on the image, I will assume and use the cubic volume, because it seems to be a giant square, like a large garden:
Volume = a³ Volume = 183,250,432,000 m³ / 1,83250432e + 17 cm³
I'm going to use the spray value, because as seen in the picture, the roses seem to reduce everything to a very small mist of dust:
Spraying = 214.35 J / cc
1.83250432e + 17 x 214.35 = 3.92797300992e + 19 J
9.38808080765 Gigatons = Island level.
Obs (Method 2) - As we do not know the total of the area that was affected by the technique, it is very probable that the real DC of this technique is Pais / Country level.
Anyway, it's a good DC feat of this character !!