Got to level 6 fairly quickly and easily then died horribly trying to get the copper penny. Doh! I don't think I can get much further without a lot of grinding xp as I'm too squishy.
You can grind some XP & associated loot by doing Town Hall Quests.
An Archer can be on the look out for Travelling Armourer to buy Hunter's Mark bow, or raid chests in Ydaya Tomb or Lannegar Mine for Hunter's Fang for a Beast Slayer bonus vs Rats.
If the spiked traps are above your ability then a few Scrolls of Detection can be good.
Bear in mind Thieves are the weakest class. They definitely have the most trouble solo but they also definitely can beat the game solo on Iron Man so it's not impossible. Just harder.
Double-check your Attributes. A couple of points in END is hugely beneficial at low level.