I guess it's a little vague, but I read it it only applies the discount once. That is also the way it works in the Ascension app. +1 for this is also how it works in the app.
For reference, it's a (9/0, monster, that transforms when you defeat it)...
If you have played a Dark Energy Shard this turn, pay 2 less power to defeat this
It can be vague, but I'm assuming the "global rule" or "global language" here is whenever you can pay a cost or something to get/do something else, it has to explicitly state that you can do that.
For example, Temple Librarian (2/1, e/h, CotG) says to discard a card and if you do, draw two cards. You can't keep spamming it to draw out your entire deck. It's used only once.
Then you have something like The World Tree (7/4, l/c IH) which says once per turn, pay up to 5 runes to gain that much honor, so it outlines that it can be done up to 5 times when you invoke it.