When I saw LEO's translation of "Fortgeschrittene" I thought "what?".

So first I checked The OUP New Oxford Dictionary of English
- "advancer" is listed as a derivative of "to advance" (= to move forwards, promote, lend [money], etc.) i.e. someone who advances something. This is completely different from the Fortgeschrittene/r, who "is advanced" (= is far on or ahead in development or progress).

The Merriam link provided by LEO says the same.

For the record:
synonyms ADVANCE, PROMOTE, FORWARD, FURTHER mean to help (someone or something) to move ahead. ADVANCE stresses effective assisting in hastening a process or bringing about a desired end <advance the cause of peace>. PROMOTE suggests an encouraging or fostering and may denote an increase in status or rank <a campaign to promote better health>. FORWARD implies an impetus forcing something ahead <a wage increase would forward productivity>. FURTHER suggests a removing of obstacles in the way of a desired advance <used the marriage to further his career>.

Ein Fortgeschrittener, as we all know, is someone who is advanced in whatever the context requires: an advanced learner (Collins German Dictionary), but also dancer, reader etc.

I propose that the entry be corrected! I've only proposed "advanced learner" as a translation, as I can't think how to make a sensible entry for a multiplicity of possibilities without listing each one.