Another tactic I used at about the same level, and gets you more XP.

The guards are staggered around the courier.  Pick up the group and get behind them in stealth.  You'll gain on them even in stealth mode.

Sap the last guard in line and wait until the group moves on.  You can kill the one guard relatively easily, cheap shot, back stab etc...  Repeat until you have just the courier and one guard left.  Sap the courier the last guard will be in front of him and he'll just stand there, go around the courier, and take out the last guard, then turn and take out the courier.

Did this to complete the quest, and have periodically done the same just to get extra XP.