This quest mainly happens at Treva's Watch. It is the second time that you have to increase your reputation at the thieves guild to do more important quests. You can find quite a few iron ingots at Treva's Watch.
I ran into two enemy mages using fireballs that killed me very quickly. However, because it is a hit or miss type of spell, avoiding it is key. I found that the best way to do this is much like I would avoid getting hit by an archer: taking advantage of the aiming system in Skyrim and move to the left and right quickly whilst walking towards the archer/mage. They will only hit you on accident if at all.
In Treva's Watch you mainly face some weaker and some tough silver hands. I found that the tough ones just hide behind their shields and let you bash them to death whilst the weaker are often one-shoted with good equipment.
