> verða skjótara að bragði

Zoega: "to make the first move"

> ef þeir færu eigi þegar að honum

Lit. "if they didn't attack him at once" ('færu' = ON 'foeri', past
3rd person subjunctive of 'fara'; 'fara at e-m' "to attack someone").

> og gera að nokkuð
> and go at some - (take some part) - [stuck there]

Lit. something like "and do something there /
in-respect-of-that-business", i.e. do your bit, take part in killing
him. MM & HP "and take part in it yourself". See Zoega 'at' II.2.
"denoting participation in", e.g. 'vera at vígi' "be an accessory in
man-slaying", related to Zoega II.1. "denoting presence at".

> að hopa á hæl

"to retreat". Yes, 'hæl' is accusative singular of 'hæll' "heel".