The Definitive Edition of D:OS2 was meant for console. Despite this, the only true difference between PC an console (e.g. PS4) is the interface.
The Fire Slugs should be level 4. Early in the game (level 4 and below) you are able to just burn everything down. The slugs are likely the first creatures you run into that have 100%+ fire resistance. This means that they actually heal from fire. That means you need to start getting creative. The descriptions of your characters sound like they mostly do physical damage. This is fine to fight the slugs. The following are basic tips for playing the game and I suspect in them you will find an answer to your problem.
1) If you have the Loremaster civil ability you can examine creatures. With this you can cursor over a creature and "Examine" it. This will tell you things like the amount of physical and magical armor a creature has, the creatures health and if it has any talents. It will also show the resistances. Any resistance that is 100% or higher will heal the creature if it takes that kind of damage. Some creatures take bonus damage to a specific type of attack, using that can help finish a creature off quickly.
2) Levels do matter. Each level you get a minimum of 2 attribute points and 1 combat ability point. By increasing the attribute that your weapon scales with, that weapon will do more damage. As such, just leveling should increase your damage from attribute points. Combat abilities can also increase damage as well. If you are struggling with an encounter and the creatures are higher level than you, it is likely best to get some XP and revisit the creatures later.
3) Gear matters. If you are level 5 or 6 and still running around with little to no gear or your gear is still level 1 or level 2, you should look into purchasing some gear from vendors. If you have a character with the "Soldier" trait, talk to the lizard in the elf cave and you should get a pretty good discount on gear. Try to keep gear within 3 levels of your current level. There will occasionally be items that fit your character build so well you will keep them for 4-6 levels, but these are the exception and not the rule.
4) Explore, explore, explore! You can get a good bit of XP just by going to new areas. This is free XP. Take advantage of it. You don't have to start fights in the new areas, just go as far as you can without starting any. Talk to everyone. This should give you the a good number of quests to complete that will net you some XP. As a new player it is EXTREMELY recommended to have the Pet Pal Talent. There are quest through out the game you get from interacting with animals. Some animals can even give you clues about solving puzzles you run into. The Pet Pal trait can get you the "Finding Emmie" quest (talk to Buddy just outside the elf cave) which is good for some easy XP. This can be completed in parallel with tip 5. Emmie is in the room with the dogs mentioned below. Just be certain you have the "Finding Emmie" quest and a character with Pet Pal opens the door to the room with the dogs.
5) Early game XP at Fort Joy : "Withermoore's Soul Jar" quest. Hopefully posting this doesn't get this nerfed. In the elf cave, there are a couple kids playing. Talk to the kids and play hide and seek with one of them. This will take you to a trap door in the cave. Below the cave you will find an undead, Withermoore. Talk to the Withermoore and get his quest. PAY CLOSE ATTENTION TO WHAT THE WITHERMOORE SAYS. Serious, write down the dialog if you have to. This should take you to the statue near the gates you entered Fort Joy (way point Fort Joy - Square). Press the button on the statue and go down the trap door it reveals. Follow the passage and unlock the way point. Go through the door. You are now in the dungeons. There is a back way to the Fire Slugs from the dungeon. In the dungeons, do not start any fights. There is a shorts stairs with a door at the top. You can pick the lock or sneak into the room with the magisters and get the key there. I believe it is on a chair nobody is sitting on or looking at. The door will take you up to the basement of Fort Joy. You can go around the basement and loot a bunch of chests, vases, etc and get some level 4 gear. I don't recommend interacting with any dogs or Magisters. Just sneak around and loot. Once you have looted, go back to the stair that got you here. If you go past a room with dogs, there is a stairs up. Go around to the side of these stairs and you should find a hidden switch. Go down through the trap door the switch unlocks. At the end of a hallway is a room with 5 jars. One of the jars will be the jar Withermoore wants. The other 4 jars have less than desirable results. Read the labels on the jars. If you remember the conversation with Withermoore OR you wrote the conversation down, there is a clue to the correct jar. Otherwise, it is trial and error. Two of the error will be level 4 fights. If you are not level 4 or higher yet, it is best to save before taking any of the jars to be safe. Get the correct jar. Go back to the room with the dogs. This time take the stairs up. This will take you into the fort of Fort Joy. Loot the room and you should get some more level 4 gear. You have a bit of room to explore the fort, the basement and the dungeon of Fort Joy without getting into any fights. Doing so should earn a good amount of exploration XP and some level 4 gear. Once done return to the Withermoore. If you haven't completed the quest yet, talk to Withermoore and complete it for some more XP. Withermoore will collapse dead and you can loot him for a fantastic early game belt. Once you get to level 5 or higher, you can go back and do the incorrect jars for a bit more XP. Loot the room fully when you return.
6) Class builds matter. You seem to have an idea of what you want your characters to do. Try to keep characters focused on one or two combat abilities as more points in a single ability allows you to use higher level skills later in the game. That being said, you can spread a few points into skills that are not your main to pick up spells that fit your build/character concept.
7) Need funds. Combine a shell with a long stick to make a magical staff you can sell. Combine a skull with a short stick to make a wand you can sell. These 2 should help you get some extra coins for buying new gear. Keep an eye out for other crafting items you can combine and sell for profit.
Other than the 2 specific quests mentioned, the above tips are valid through out the entire game. Finding and completing as many quests as possible should keep you leveled up through out your game.
Good luck and enjoy a game that many have before you!