1) Do what you can to always hunt higher level monsters. Don’t settle on equal level monsters if you want to become good. Even if its 1 level above you, the extra bonus experience will help immensely in the long run. Not all classes can do this equally well, but with a decent supplementary glyph (spell), all classes can do this, versus the right monsters more often than not. Pick targets wisely, learn to realize when you can kill even monsters 2 or 3 levels above you, with the right combinations. Do power leveling. Then bosses will be nothing.
2) The first point is also doable by taking advantage of the tile resources to gain some health and mana in some cases. Although monk is possibly the best in this (insane tile regeneration and great in killing physical damage monsters), you need to be able to identify situations in which you can regenerate just a little more health and/or mana to finish off something. Everything can be calculated. Each monster regenerates its level in health, per tile you explore. This is true for you as well, although your mana only regenerates at a 1 point/per tile rhythm.
3) Take advantage of deities, they can give you the upper hand. Some are complex to fully understand with all their likes/dislikes etc, and might take some time to feel comfortable with, but in most cases they help a run immensely. Experiment. Logbook and wiki can help in faster acquaintance with them.
4) Buy an item from dungeon shops to help your early leveling if possible, take stuff from your lockers and stuff as preparation to make dungeons even easier if you struggle.
5) Do catapult leveling on lvl 8-9 to kill the Boss(es). If you do it right you can kill a level 10 boss at the lvl 8-9 range without a single potion use, by throwing all your spells and attacks on the boss, leveling via forgotten low level monsters on the map that you can one-hit kill, then continuing after the level up with all your attacks and spells again. Its possible to save your potions to kill the 2nd boss that later levels have. Easy.
6) Tile exploration is the main resource. That’s how you get HP and Mana. Take advantage of it the most that you can. Don’t be wasteful its the more precious resource.
7) Take your time, this game has deeper gameplay than one might think. Its mostly always winnable, the only way to lose is by being in a hurry and not thinking enough.
Its an insanely well thought-out game, don’t let others fool you into thinking that it’s luck based. Nothing could be further from the truth. Its the exact opposite.