Keep casting the magic arrow spell even after running out of mana. Stop casting the spell when your character's current and maximum hit points have gone up. You character's constitution should also be increased.
You can save just before entering a level in order to load and reload that next level to hunt for valuable items. The best item to hunt for is a Wand Of Clone Monster. Save a couple Wands Of Clone Monster. When you get to the levels with Hrungnir in Castle 1, or the Giant Kings in Castle 2, clone off the Giants however many at a time you can without getting killed in the process. When you kill off the cloned Giant Kings, you will get massive amounts of treasure.
Continuously cast any spell until your mana reaches approximately -8000. Once done, the mana should stop decreasing when you cast a spell. Then, cast a healing spell and it will add to your HP until reaching 32767. Note: If you cast more health spells after you reach that number, your HP will turn into a negative number.
Create a character and choose Detect Item as a first spell. Go to the dungeon, and save the game prior to descending a level in this or any dungeon. Enter the dungeon and cast Detect Items. If the items revealed are not desirable (spell books, armor, weapons, money), load the saved game and repeat. Since a different dungeon appears with each load, valuable items will eventually appear.
Turn off visual effects in the options section under the file menu, then use a spell that raises your health. Use it until the screen stops showing your HP. After that, cast an attack spell (fireball, etc.) You will get a few thousand HP.