The combat in Legend of Grimrock 2 is more difficult than the one in the first game. Many enemies are different among themselves in the sense that they will employ different tactics to fight you, delay you, or even escape. For instance, a giant frog may leap over your group to flank you, while wargs move with extreme speed. It's important to understand how these monsters behave so that you can take them on successfully.
One important aspect to consider while playing this franchise is that you should not have your party stay too long in front of an enemy, lest you be damaged. You will need to combat-dance whenever you get the chance to minimize your damage received. Even a good combination of champions will very likely be heavily punished if you do not use this technique. Basically, you should wait for a monster to step on the square in front of you ;preferebly if they come from either left or right so that they are not facing you directly; then you may use as many attacks as possible, and following that you should move to another square before the enemy can hit you. Rinse and repeat. Once you get used to this method, you should be able to take the offensive more skillfully and gameplay becomes smoother.
I remember not combat-dancing when I started playing the first Legend of Grimrock and the game was much harder; I just stood face to face with an enemy until one of us died. Once I realized I could combat-dance, everything made sense and I finally felt I had the upper hand.
These are just some hints that I hope help improve your experience!
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In terms of actual party creation, remember that usually the two champions in the front row will be the ones receiving more damage most of the time, though your other heroes may still be injured if you are flanked or hit by a spell/trap. Nonetheless, the two front characters are the ones at higher risk and, therefore, should be prepared to deal with large amounts of punishment. Even with the combat-dancing described above you are still bound to be hit sooner or later. You should start by selecting classes that are more prone to survival than others for your frontliners, such as Knight or Barbarian. When allocating attributes, traits, and skills, try to follow the same thought pattern and grant them abilities that improve their survivability, such as protection bonuses or more health.
I find alchemy very important in this game, as potions and bombs are extremely useful, so I would recommend having an alchemist in your backrow for the class-ability to grow plants in your inventory, which in turn means you are able to brew more potions and craft more bombs. Don't forget to allocate points in the alchemy skill as well. Your alchemist will still need to deal some damage, and you're able to choose what sort of attacks you'd like him/her to perform. Missile weapons should be a decent bet, though it may take some time until you come across a bow or crossbow. If you decide to take magic with this character, such as Fire or Air, remember that this class needs a specific equipped item to cast spells, such as a wand or a staff.
For the sake of variety, I would have the fourth character be different than the alchemist above. If you make the alchemist a ranger, make your fourth character a battle-mage. (Battle-mages and wizards are able to cast spells from the get-go, with their bare hands.) If you make your alchemist a spellcaster, make your fourth character a ranger. This also means there will be less competition for items and everyone can profit from your finds.
Note that these are just some suggestions and tips to help you start. There are numerous successful party combinations you can assemble.