Depends how many.
Solo is a bit tiresome - it takes a long time and you have to cheese it. 2 or 3 is the best number.
For 2 I would play Inquisitor/Necro focusing on INT and using a staff. Take one point in every skill and get all the close range magic you can. Add 50/50 memory and INT. Back it up with a ranger. Don't mess about with this - put every single point into ranged and FIN. The Inquisitor is more fun but the Archer will do more damage than anything else you can get.
For 3 I'd do the same and then add either add a scoundrel (if you like that) or a STR based character. Scoundrels do more damage but are weak early on. Playing a STR based character with dual wielding swords does a reasonable amount of damage (better than a two hander) and counters the dullness of the archer pretty well.
You don't need a mage or a healer. Their attack isn't strong enough and you don't need healing as there are plenty of potions and you can use Necro or Hunstman skills anyway.
If you want 4 characters you could add anything (and have less cash for equipment) but 3 is the magic number.
You can make a custom party like this
https://steamcommunity.com/app/435150/discussions/0/343787283759112190/?ctp=2Essentially you start one instance of your game and set multi-player. You then open new instances and join it over LAN. Memory will be your issue here. I have only 8GB and can't get more than 2 instances to concurrently save even if I turn graphics to minimum. You may have more luck but the number of custom characters you can make depends on how many instances you can save at the same time and that is memory based.
Once you have saved though you can play them all as one.