I suspect "arduis" is right, and yes, it would be the ablative plural of "arduum" = something steep or difficult. The meaning would be something like "by difficult feats, to the sun". I would have trouble construing it with "-us", but what do I know?

*In Latin*, the o in "bonum" (neuter singular of "bonus", good) is short. However, N.B., when Latin words are taken bodily into English, the classical quantity of the vowels has (almost) no influence on their pronunciation in English; instead, long vs short is largely determined by the ending. "Bonus" is a handy example: As an English word, it has long o. For a discussion, see Fowler's Modern English Usage s.v. False quantity.