The solution was there, but it just clicked in Cane's mind when he prevented Lou from using his magic by restraining him with the black wisp from the lycan.
Just like how it worked with the letter from Rei that was sealed by black magic, by using the black wisp, Cane also was able to neutralize the black magic that was used to create this barrier around Winter Moon pack.
The shield was still there, but the black wisp neutralized a huge part of the black magic and allowed the warriors, knights and the magic users to go through it. It didn't matter how strong the protection shield was, because once the magic users, who created it died, it would disappear too.
And with the black wisp neutralized the black magic, they finally could really start this war.
