A reminder this game draws HEAVY inspiration from Darkest Dungeon, but unlike DD, it doesn't have a death-spiral. Dungeon supplies do not cost any gold, and abandoning a mission will still pass the day, so the tavern stress reduction and other daily-things tick over. in DD, you could doom yourself by abandoning or losing too often, to where you couldn't get any supplies, new recruits, or stress-relief. This game doesn't have that. Battle going poorly? Dungeon only half done, and out of morale for health refills?
Retreat. You keep everything you found so far, and may get some negative traits. Who cares, you only get a max of 5 negatives, then they override eachother. Pushing to the end of a rough dungeon may actually be worse than retreating, due to enemies being able to boost your lewd crest scores at high stress levels.
Get a solid front-liner, a dodgy or tanky number 2, then a cleric (or mage if you feel like gambling) and damage dealer for 3 and 4. Avoid fighting as much as able until you find the exit, then work through them easiest to hardest by enemy type seen on the map.
Honestly, just avoid slimes if you can. They are beefy, auto-block, when they merge all damage dealt to the merged ones is useless, and all the slime variants are a PAIN (if more than 2 fire slimes are in a enemy party, just retreat) to fight. Ratkin are pretty predictable, just focus-fire the morters, or force them into position 1 repeatedly.
The game punishes getting greedy and over-reaching. "I can take a few more deaths-door hits" is not a mentality you should foster. Unless you have teams you are perfectly ok with just losing, AND the items they have equipped, you should be trying to retreat on your first or second death's door warning (depending if it was a lucky crit or not). There is no time limit, no cash-crunch, no slow drain or minimum profit needed per day. Also, power-level the everliving fuck out of your wagon and your maid-capacity, get those max-stat newcomers and huge morale pool for dungeons, it'll change everything.