Hello +Matthew Callison ,
if I am not mistaken, the intended meaning is this (it is always a good idea to specify what you actually tried to translate:):
Life without death
Endurance/faith without me(?)
Journey without end

Although the prefix ar- can indeed mean 'without' as in arnediad 'without reckoning, numberless', I think the preposition pen 'without' fits these expressions better (attested in Iarwain ben-Adair). At any rate, ar- will cause a liquid mutation in the first consonant of the following word, so they would look like arwannath, arveth.

Cuil ben gwanath
Bronwe ben im
Lend bem meth (?)

... assuming pen causes a nasal mutation and not a mixed one (in which case the last example would be be-meth).

Any other thoughts?

Pen causes lenition, compare pen-bed (PE17:145). It probably had been nasal mutation at some point, but mutations in prefixes of this kind seem to have been analogically levelled to lenition (see also adbed (PE17:167)).
Hence I would say *pen-wanath, *pen-veth.

Thank you, +Roman Rausch , indeed I forgot to check PE17.

Excellent!  Thank you +Tamas Ferencz and +Roman Rausch  for the feedback.  I've been going through David Salo' "Gateway to Sindarin", but for someone without a linguistics background, it's a bit daunting.  What I was trying to translate was the First Ideal of the Knights Radiant from Brandon Sanderson's "Stormlight Archive":

Life before Death
Strength before Weakness
Journey before Destination

I think I have more learning to do before I can recognize a nasal mutation, but I'm jumping in with both feet and willing to learn.

Also, the second word on the second line was ARNIN, not AMIN, but I see that using italics confused the issue.

I also now see that "nin" is the personal pronoun "me".  I was trying to use "nind" for 'fragility' or 'weakness'.

Also, +Tamas Ferencz and +Roman Rausch ,  your comments led me to elfdict.com, which looks like a fantastic resource. Hantalle! (hopefully "Thank you" or "Thanks to thee")

Is the 'before' in these sentences to be understood as 'before in time', or as 'preceding in priority, more important', or possibly both?
Either way it's not going to be easy to translate and we may need to rephrase the sentences somehow - we just lack the vocabulary.
nind is and adjective meaning 'slender' - I don't think it can be used as a noun for 'weakness'.

The 'before' is used in the context of 'more important', but I could not find any sort of preposition that indicated a physical location 'in front of' or anything of 'higher importance', which is why I was going with "not".  Also, I was hoping the root word NIN for "fragile or slender" would lend itself to 'weakness', but perhaps not.

Would the prefix u- or al- be useful here?

Maybe I could use sennui and swap the order of the words and have something along the lines of:

Uwanath, sennui cuil

Well, it's a tricky one. Tolkien talks a lot (well, relatively speaking [pun intended]) about the comparative in Quenya in Parma Eldalamberon 17, but stops short of giving us anything meaningful about the same in Sindarin.

One could employ the verb aphad- 'follow' like Gwanath aphâd cuil, but I find this solution inelegant, as it switches the focus os the sentence from life to death.
Or one could use the preposition or 'above, over', in a figurative sense: Cuil or 'wanath (with liquid mutation).

Also, I was hoping the root word NIN for "fragile or slender" would lend itself to 'weakness', but perhaps not.

Well, perhaps yes; the root is NIN-DI (see Etymologies), and we see roots of a similar structure (cf. HEN-DE for example) yielding nouns, so perhaps nind ninn could also be the noun meaning 'frailty, weakness'.
The Etymologies also have the Ilkorin noun bel 'strength' (cf. Beleg), in S that could be bêl, so weakness could be *úvel, *pen-vel?

Concerning the Sindarin comparative, there seems to be also a word rogelair ?'very bright' or just a prefix ro- (ra-) provided somewhere in the later publications, so as to be presented in the grammar summary here, besides an-:

http://www.sindarin.de/grammatik.shtml#adjektive

(It kind of reminds one of the Goldogrin comparative suffix -odro(n),  #-ro in PE11:16.)

That prefix is in PE17:147, where we have "S rau-, ro- is prefix as in rovaed."

*Rogelair was my reconstrution so that I can translate elenion ankalima. But this prefix doesn’t really help here..

I think that using negation is a good idea, but one might use û, termed ‘adverb or interjection’ (PE17:144) rather than pen- ‘without’. So my suggestion would be:
Cuil, û ngurth ‘Life, not death’
Tûr, û ben-dur ‘Strength, not weakness’
Lenn, û vened ‘Journey, not arrival’ (taking *men- ‘go as far as, arrive’ from Quenya)

From the Etymologies we also have Noldorin belt ‘strong in body’, bellas ‘bodily strength’ (no need to go to Ilkorin), but I think that a more abstract ‘strength, power’ is required here.

Matthew: As an alternative solution, you can also use athan (beyond), that is

Cuil athan gurth
Hûr athan phaw
Lend athan meth

?Athan is likely a misreading for *athar by Christopher Tolkien - he in fact mentions himself that the word is hard to read. *Athar one could at least relate to the root THAR- 'across, beyond', and the absence of nasal mutation would also make sense.
In any case, using it as above would somehow imply that bronwe athar harthad means 'endurance before/instead of hope'...

True, but "beyond" has many shades of meaning (cf. e.g. the phrase "sg is beyond sy’s wildest dreams"), so I think athan(/r) is a good candidate for expressing comparison.
 
Btw, *athar (from THAR) would cause lenition (cf. tharbad), so we should also expect *charthad instead of harthad as in the case of using athan.