I recommend you to use company-mode for auto completion of Elixir programming.
I created ac-alchemist which is auto-complete source of alchemist. Because alchemist provides only company-mode completion.
alchemist often changes its API frequently. So it may break ac-alchemist. Please report me if ac-alchemist does not work with latest alchemist via github issues.
Repository
https://github.com/syohex/emacs-ac-alchemist
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