The Vikings never wore horned helmets into battle, but they did drink from horns. When a Viking warrior died in battle, he was buried with his drinking horn. When people/archaeologists stumbled across Viking burial grounds, they found the horns and just assumed that they had become detached from their helmets.
Ashildr says, "We will be cut down like corn." Although "corn" (maize) is native to the Americas and was not introduced to Europe until the early 16th century, the word "corn" has been used in Britain since the Middle Ages as a generic word to refer to cereal crops such as wheat, barley or oats. As such it would have been the natural choice for the TARDIS's translation system when translating from Old Norse for the benefit of Clara and The Doctor.
Straight (unadulterated) testosterone can not be taken orally because the liver readily deactivates it. Of course, that's in humans, not Mires which probably have different abilities.
Electric eels inhabit fresh waters of the Amazon and Orinoco River basins in South America. It is highly unlikely that the vikings would have had access to them.