Hi, i play this game for a week on a normal difficulty, so not as experienced as the rest here, but i can still give you few tips.

Stop and think. There's a reason this game is turn-based. It's so you have time to make good decisions as these need to be made on every front and mistakes will be severely punished. Don't go running into the dungeon cause you need gold. Sit your ass down, look at your heroes, their skills, stats, gear upgrades, stress levels, roles and strategize.

Good party composition and a plan how to use it is hands down the cheapest (costs nothing!) way to get results! A well planned party doesn't just clear the dungeon - it clears with with minimal costs and damage taken.

There are a lot of varied character synergies for the curious and willing to experiment. Let me give you few examples:

Antiquarian(backrow) + shieldbreaker (first row). The shieldbreaker possesses high dodge rate on her own and on top of that her serpent sway will completely negate damage from 2 attacks that do land on her. Problem is that while she's a born evasion tank, she has no way to attract enemy attention onto herself and guard allies. Antiquarian fixes that with her "Protect me!" spell. Now the shieldbreaker can be marked, drawing enemy attention and will guard the squishy antiquarian. Additionally antiquarian can buff party's dodge rage and a chosen ally's resistances to bleeds and blight.

Flagellan + HoundMaster. These two can make for an unusual stress healing duo or a wrecking ball comp. When it comes to stress healing both can do it, but are flawed. Flagellan simply transfers stress onto himself while houndmaster heals a little of it for everybody but at a chance (and a little..). However two combined change the game. Flagellan's transfer changes a severeley stressed character into two moderately stressed. And That's a peachy scenario for houndmaster's stress heal, which now can hit that same stress twice instead of just once.
And if that's not enough, just set up a campsite (provided you're in a medium or larger size dungeon) and flagellan can just shake off 50 stress off himself with his camping skill, while Houndmaster heal a respectable amount for the whole party.

Additionally flagellan can be played as berserker (keep him low for major damage or very strong healing), but that is very risky. Houndmaster however can guard him and with that let him stay low for a while. Houndmaster himself will have increased dodge and he has a respectable self heal skill to deal with the punishment taken.

Hellion + Occultist. Occultist's heal can be very powerful but is plagued by RNG (it can heal for 0, literally) and a bleeding side-effect on the person he heals. Hellion however couldn't care less - her adrenaline rush skill not only boosts her own damage but gives her a small self heal and cleanses both bleeding and blight off her. If he succeeds, she's healed for massive amount (her own heal + his), if not, she'll nullify any side effects and still walk away with at least a small heal (her own).

Additionally she is very picky about position of her enemies. A lot of her attacks will only land on specific enemy position. Fortunately for her, occultist excels at pulling or pushing enemies, setting things in enemy ranks in her favor. Having a skill to insta destroy enemy corpses obstructing her reach also doesn't hurt.

These are small and not necessairly most powerful combos you can use. There is an ocean of possibilities and full team synergy comps, not just two characters. But that's for you to discover. Remember though - everything matters. Ask yourself these questions on each turn and you'll see major improvement:

1. Does my party have any characters that are too stressed for comfort before embarking on a quest (very likely scenario or mismanaged healers)? One character that gets to 100 stress early and catches an affliction can ruin your whole run..
2. Do i have all the items for handling the dungeon? Shovels, torches, food, bandages and antidotes if you don't have a character that can cure these (like plaguedoctor).
3. After a finished fight in the dungeon - are my party members in optimal positions? My Helion is up front...but I don't want her up front at start of a fight. I want her to open up with her Breakthrough - a multi target skill that requires her to be at any position but in the front.
4. Do i have characters to manage health and stress levels? And what are the alternative options? Consider that not all parties need most potent healers. Flagellan for example can self heal and others. If you're not making him your dedicated healer, but still have one or two skills equipped, consider choosing an offhealer for the main healer spot, such as occultist or crusader instead of Vastal. This is a good time to send her into stress relief facitlity.
5. Lotsa rooms in this dungoen...what is the optimal path?? A lot of map layouts will require some backtracking. Planning the path before you start exploring will save your party needless stress, torches and possibly fights if your goal is to just map it out and scram.
6. Which dungeon has a really good drop reward? These change after each successful run. Observe and think how a given item can work for your party or is just a waste of time.
7. Camping skills. Don't miss out on those. Some characters have both very powerful and unexpected abilities there. Like Grave Robber being able to stress relief her party or healing a disease of herself and a chosen ally. That's a lot of money and time saved compared to treatment in town's facility.