Although de4dot supports a lot of obfuscators, there's still some it doesn't support. To decrypt strings, you'll first need to figure out which method or methods decrypt strings. To get the method token of these string decrypters, you can use ILDASM with the 'show metadata tokens' option enabled. A method token is a 32-bit number and begins with 06, eg. 06012345.
This command will load assembly file1.dll into memory by calling Assembly.Load(). When it detects calls to the two string decrypters (06012345 and 060ABCDE), it will call them by creating a dynamic method, and save the result (the decrypted string). The call to the string decrypter will be removed and the decrypted string will be in its place.
de4dot file1.dll --strtyp delegate --strtok 06012345 --strtok 060ABCDE
Since the assembly is loaded and executed, make sure you run this in a sandbox if you suspect the file to be malware.