I have real big doubts about your claim to be underleveled after completing everything you can, but lets just say for the sake of it that you are, you somehow failed every quests by mistake and that you builded a warrior with all stats in Intelligence. You can still complete this game.
If its too hard, you can switch to easy mode mid-game.
You can rebuild your characters stats, google how to and just look at some good builds, there are some pretty broken ways to play this game where 1 shotting bosses is possible.
Learning the crafting system will make your gear so good that you can be 5 levels under and you wont even notice it, you can also make loads of money with it and buy the best gear from shops.
Most bosses have strategies to defeat them or ways to cheese them. You can pretty much cheese everyboss by stacking ooze barrels and other such ways, so your level is not that important. Again google is your friend.
Personally, I think DOS1's difficulty is childish even on harder difficulties. If you have learned how to gear properly using crafting, learned how to build your characters, looked up strategies on how to win battles and still cant beat DOS1 on easy mode, then maybe there should be an auto-win mode where you can win no matter what you do.
