In Japanese it's called: アルセウス (A-ru-se-u-su) (Pronounced as: A-ru-se-us) It's name comes from; possible 'arch' (highest) and 'Zeus' (the ''most powerfull'' of the Greek gods. (or 'deus', meaning 'god' in Latin). Still for me it seems the word; 'Arkhaios' (Αρκαιος), Classic Greek for 'beginning', is involved in this too. But I don't think they went this far for searching a good name.
Anyway let's base it on the Japanese... We, in English, don't pronounce the 'u' after the 'r'. So 'A-r-say-us' or whatever would be impossible. That comes to the conclusion the first syllable is 'Ar'.
The 'ce' part shouldn't be pronounced as 'kee', in Japanese it isn't either, so let's state it's pronounced as 'see'. Yes, I changed my mind about it, instead of 'say' I would prefer 'see' now, explanation will come later. (Yes in Latin it would be pronounced as 'kee', but it's still a based of something Greek so I don't prefer to involve Latin in this).
So we have; 'Ar-see-...'. And the last part should be pronounced something like 'yus'. As you can see there's the 'y' I removed from the 'say' what is now 'see'... Anyway, it fit's too with the pronounciation of 'deus' (Latin for 'god'), which is (probably) pronounced as 'dee-yus'.
So yes after this looong explanation we can conclude it would be pronounced like 'Ar-see-yus'. However Pokemon Battle Revolution still pronounces it as 'Ar-see-oos', in the film they say; 'Ar-say-us' and Pokemon.com said it was 'Ar-kee-us'...
Still I prefer 'Ar-see-yus'.