This challenge starts off with an encoded file, “enc_flag” which was downloaded with wget into Kali Linux Virtual Machine.
Further reading from base64encoder confirms this and explains that a double == instead of single = means the last chunk of the message is exactly 16 bits. 
Hence ASCII decoding was performed on the message using bash command.
echo -e <string> | base64 -d
This did not convert the message into any recognizable English. It may be that a different encoding was used but the clues in the hint suggests a different reason. The wording was “multiple decoding” with no mention of different decoding schemes stacked together.
Hence the next step is to take the output of this decoding as input to a second iteration of decoding, then doing this repetitively till it becomes legible English. To prevent wasting time if this is the wrong approach, I will run no more than 10 iterations.
Upon the 5th iteration, the output has shown the flag.
 “What is Base64 Encoding and How does it work?,” www.base64encoder.io. https://www.base64encoder.io/learn/