You can only use the sanguine elixir to aid in taming when it is in the last hotbar slot, and you're crouching, just like when you feed the fish basket. Do *not* for the love of god, aggro it if you're using a trap, because even if it doesn't lose all taming progress immediately, it will never stop aggroing on you or the trap until it *does* lose all taming progress. If you aggro it part way through and it doesn't say you lost all progress yet, don't bother trying to save the progress.