Here are some useful phrases.

• Ungi maðurinn þekkir reglurnar, en gamli maðurinn þekkir undantekningarnar.
  “The young man knows the rules, but the old man knows the exceptions.”
• Nú duga engin vettlingatök.
  “No more mitten-takes.” — used for: Do it right this time.
• Ég kem alveg af fjöllum.
  “I come completely from mountains” — used for: I have no idea what is going on
• Ég mun finna þig í fjöru
  “I will find you on a beach” — used for: Don’t make me hurt you. Sort of.
• Að koma einhverjum fyrir kattarnef
  “To put someone before a cats nose.” — used for: Putting someone in the spotlight, but slightly more dangerous.
• Ég borga bara með reiðufé
  “I only pay with an angry sheep” — used for: Okay, but I don’t have to like it.

Edited to add: I put in rough ideas of how one would use these phrases