You need to power up as much as possible via conversion, spells and equipment.
While working your way up to level 10 (or as far as you can get, as you often won't be able to hit 10), you'll want to fully utilize health recharge from leveling and exploration (waste as little as possible) as well as experience bonuses from defeating higher level creatures and using certain status spells.
As The Avatar stated, you don't need to be level 10 to beat the boss.
Bottom line: The game is hard. It's a bit like chess where you need to think ahead and scrutinize every move you make. If you are having trouble, slow down, think, and maybe phone a friend =)
There's a quote on the front page that speaks volumes about the game-play:
"Where I most enjoy DD is the moments where you seem stuck in an impossible situation, and the everything-must-go desperation which results."
PS: If you think the regular levels are hard the challenges found in the Explorers' Guild will blow your mind! (really whether or not you think the regular levels are hard, actually ^_^)
Catapultam habeo. Nisi pecuniam omnem mihi dabis, ad caput tuum saxum immane mittam.