My first advice, which is going to be useful no matter what floor you're on, is to get used to how far your sword swings, how quickly you can attack, and to start using the "parry" secondary attack that swords possess if you haven't already. Ideally you want to hit enemies as soon as they enter your range of attack, and keep them there until they die. If your sword swings quickly enough, a good idea is to place yourself in a way that will push enemies against the wall, letting you stunlock them to death.
Don't be afraid to use food to restore health, it is one of the main reasons you'd want to cook food yourself, and Salads (available at Gus' saloon) offer a very good HP/gold ratio from the start of the game. Given how deep in the mines you are, you should be able to buy Dark Boots at the Adventurer's Guild (2500 g) for 1 more Defense but 1 less Immunity compared to Firewalker Boots, which is going to be a nice help if you find yourself getting surrounded by mobs of enemies. Crafting or obtaining defensive rings like the Vampire Ring (from killing 200 bats), the Sturdy Ring (recipe unlocked at level 1 Combat), and the Ring of Yoba (recipe unlocked at level 7 Combat) can help with staying alive a little more easily.
Speaking of Defense, the Autumn's Bounty, Pumpkin Soup, and Stuffing recipes could be available to you depending on your friendship levels with the NPCs, and given you have upgraded your house at least once to unlock the kitchen. All of those will increase your Defense by 2 for around 8 minutes in addition to restoring health and stamina, which should help when you find yourself in a sticky situation.
Keep in mind that the magic projectile that the shaman-looking enemies fire at you will lower Defense by a massive 8 points, so I suggest running away from enemies if you happen to get debuffed from that, and waiting until it goes away.
If all else fails, you can simply mine through as many rocks as you can while avoiding enemies until you find the stairs to the next floor; you don't have to fight enemies at all if you just want to make progress in the mines. If you have a decent amount of Stone you don't mind parting with, you can just craft Staircases from 99 Stone each. 495 Stone will get you from one elevator to the next without having to fight a single enemy or mine a single rock.
Lastly, you really need to upgrade your pickaxe. Even if you decide to never delve past floor 90, going from 2 hits to break a rock (or more!) to just 1 will save you a lot of time and stamina when looking for ores, and you will want to look for them for even more sprinklers.