Help Overwriting the META INF without Forge

[Amaxter]

So I've made a modpack recently that makes installing a non Forge mod for Minecraft easy. I compress all the class files inside a modpack.jar but it won't launch and gives me this: http://pastebin.com/uVZ3QCKf . I suspect it has something to do with the META INF, and I tried to overwrite it by replacing the MANIFEST file with a blank text file with the same name and extension but it doesn't seem to work. Does anyone know how I can overwrite the META INF as I can't delete it since Technic downloads all the minecraft jar files?