> þá skuluð þér þó fara að honum oftar.

MM & HP have "again and again", but I wonder if it would be enough to
translate the clause as "then attack him again", as 'oftar' is often
used where we might say "again", or in the negative also "any more".

http://www.lexis.hi.is/corpus/leit.pl?lemma=oftar&ofl=&leita=1&flokkar=Fornrit&m1=oftar&l1=Leita&lmax=1

> það mundi honum bráðast til bana

"it would [lead] to death for him"
"it would very quickly be the death of him"

'bráðast', superlative neut.nom.sg. of 'bráðr' "quick, sudden".

> En ef honum vill þetta til dauða draga

"But if luck will have it (if it's so fated, if it happens to be) that
this will lead to his death..." In Grettis saga, Glámr rounds off his
curse on the hero with, 'Ok þat mun þér til dauða draga' "And that
will lead to your death", "...prove fatal for you", "...turn out to be
the death of you." (See the right hand column of Zoega p. 91,
impersonal uses of 'draga'.)

LN