Emilia creates 6 layers of ice walls to defend against Shaula's attacks in Arc 6.
Arc 6 Chapter 12 - A Storm from the Tower
For the length of each wall, I will assume 10 meters (from 'several tens of meters' tall ice tower laid down side ways).
L = 10 meters
I will give the ice walls a height of 4 meters, because Emilia (1.64 m tall), who is covered by the wall, is on the roof of the dragon carriage creating them. (Image 2 and Image 4 below)
Dragon Carriage is this tall. The wall is taller than Emilia's height added to this.
H = 4 meters
For the thickness, I will assume the slender ratio of 0.25 for solid medieval defensive walls. That's 1 meter thick.
The actual wall is actually extremely thick.
W = 1 meters
Lastly, Image 4 also shows that the creation is instant.
Let's run the calc now.
6 Ice Walls Volume = 6 * LHW = 6 * 10 m*4 m*1 m = 240 m3
Ice Density: 916.7 kg / m3
240 m3 *916.7 kg / m3 = 220,008 kg of Ice
Water Specific Heat: 4.186 kJ / kg C
20 C to 0 C
220,008 kg * 4.186 kJ / kg C * 20 C = 18.41906976 GJ
Water Vapor -> Ice Latent Heat of Deposition at 0 C: 2845.12 J/g
220,008,000 g * 2845.12 J/g = 625.94916096 GJ
Total ice creation energy: 625.94916096 GJ + 18.41906976 GJ = 644.36823072 GJ = 154.007703327 tons TNT or Multi-City Block level
Ice Deposition analysis:
Water vapor in the air is directly deposited into ice. (water vapor -> condensed water -> frozen ice yields higher calc)
Similar example of Deposition from the same anime:
https://www.youtube.com/watch?v=-dgsQuV9iHY
Other examples of Emilia's large-scale ice creation feats in Arc 3:
Arc 6 (more powerful Emilia) ice creation feats are broadly consistent with earlier ice creation feats performed by a younger Emilia.
(The calc page had value for Heat of Fusion and Vaporization of Water for freezing water vapor in a 2-step process, but did not have one for Heat of Deposition, so I looked for it on another site: http://vsbattles.wikia.com/wiki/Calculations)
The 2-step process is 2909.16 J/g, so it would have slightly increased the result compared to Deposition.
Lina Shields made another calc of the feat in the comments below with the same AP tier result. Emilia is Multi-City Block level or 8-A.