Long time DnD player/DM here (3.5/5/pathfinder), I've got about 7 hours in the game so far on 'classic' difficulty but have found almost every combat encounter after arriving at the Druid's grove has been pretty much impossible to beat. The temple near the grove had a fight with so many level 3, perhaps 4 skeletons with casters that I decided it must be intended to return to at a higher level.
I did this fight solo at level 2 on classic (its the only available difficulty option) with a Wizard. It is definitely a you problem.
Inb4 people say git gud, however there seems to be little chance I could return at a higher level as xp gain is fairly slow and I've yet to find any side quests with an appropriate challenge for the party to gain said xp. Party composition seems to make little difference and I know as a DM for dnd that if I ever threw similar encounters at a party of lvl 1s, I would expect them to run away as it's certainly not winnable.
somebody please tell me I'm missing some significant mechanic or quest tree, as I suspect coming back with a level 2 party would have the hit points to survive it.
Saying "Git Gud" is not very helpful so no, instead I'll try suggesting things which can help.
1. Use line of sight to give you an advantage against ranged enemies. If they cannot see you, they cannot hit you. Hide behind walls, in enclosed rooms, etc and restrict how many enemies you fight at a time. This was how I did the fight you complained about further up, when the casters appeared I just ran into the room, cast grease at the door and then used cantrips like Ray of Frost on them as they walked in.
2. Use height (when its an option) to your advantage. Stand on high ground, shove off any enemies when they come close and then use ranged powers or a bow against them, as you will have advantage.
3. If you see someone hostile to you at a distance and you know moving closer to them will engage combat, enter stealth, position your characters appropriately and then use sneak attacks to initiate combat. This will give you a surprise round which will allow you to alpha strike them.
4. Make use of crowd control effects and appropriate buffs for encounters.
