They are the last trophic level in some hierarchies because they feed on everything (National Geographic).
However according to the strict trophic level definition they would be primary consumers.
They are the last trophic level in some hierarchies because they feed on everything (National Geographic). However according to the strict trophic level definition they would be primary consumers because they consume a source produced by natural cycles like plants.
Unless the organism actually invades attacks or otherwise causes the death of another trophic level source it cannot really be considered to consume that form. It is consuming something different than the original creature. See the following diagram.

See also the following helpful sites:
http://www.eschooltoday.com/ecosystems/ecosystem-trophic-levels.html