In this case, the full phrase would be "il est impossible de nous relever", similar to the English "impossible to... " being the shortened version of "it is impossible to...".
"c'est impossible" better matches "this is impossible", so it does not work as well in that sentence.
In the case of the adjective describing a situation, I have been led to believe that "c'est" is to be used rather than "il est". And when the adjective describes a person, "il est" is used. It seems like the "impossible" aspect here describes a situation, not a person. I also understand that when impersonal expressions are used that one could use either "il est" (more formal) or "c'est" (less formal). I understand you are saying that "c'est" is more like "this is" rather than "it is" although "c'est" is often translated as "it is". Is the "c'est/il est" rule I have described valid? http://french.about.com/library/weekly/aa032500.htm
"il est + modified noun" has to be changed to "c'est + modified noun".
- he is a man = c'est un homme
"il est + adjective" describes a man, a masculine animal or inanimate object to describe him/it with a qualificative adjective
- he is tall = il est grand
When the English is "it is easy to", "it is necessary to", "it is possible to", the French will also use "it" as an impersonal "il".
- it is easy to travel to Belgium = il est facile de voyager vers la Belgique
When the English uses "it is + adjective" where "it" represents a thing, you translate it to "c'est + adjective"
- it is easy to do = this thing is easy to do = c'est facile à faire = cette chose est facile à faire.
I also opted for the non-reflexive "impossible to raise us", as in: "grâce à notre poids, il s'est avéré impossible de nous relever", or "les necromanciens vont trouver qu'il est impossible de nous relever". I realise on reflection that it may seem a facetious and/or over-literal translation, but is either the physical/metaphorical lifting of people or the awakening of the dead/sleeping a context in which it would also be an accurate one?
I don't understand what the sentence is trying to say. I translated it "correctly" (by Duo's standards), but what does it mean? Is it saying "we're unable to get up again"? As in, "we've fallen down (again) and can't get back up"?
The word "impossible" is throwing me a bit as I don't think it's translating well into English in this sentence.
"on" does not have an object or stressed form but its reflexive is "se" (like "il, elle, ils, elles"). So "on va se relever" is correct either with an impersonal "on" or as a substitute for "nous".
What you cannot do with "on" is a construction like:
- elle nous a vus (= nous - direct object)
- elle nous a parlé (= à nous - indirect object)
- elle a parlé avec nous (= nous - stressed pronoun)
Considering how pernickety the French are about their prepositions and pronouns and articles, frankly this is appalling English. You might be able with twisting your logic to come up with a scenario that supported it, but frankly this needs to say 'It was' or 'It is impossible..'