Lowering difficulty is not a option as I do not ventured outside the town.
As for using guides or walkthroughs, I rather uninstal this game and never touch it again.
Using external help with invalidate the experience
Listen. You don't have to read everything in spoilery detail.
This will give you a list of "parts". You are at part 2; the investigation. Part 3 is suggested as "gathering strength at the black cove".
"You've now resolved two-thirds of the undead problem. By resolving both The Lost Archaeologist and The Scaredy Pact, you've investigated the burial mounds and the lighthouse, but have come to no avail. You're no closer to solving the murder either, but the answer lies to the east at the old church. However, the church is very well guarded and you are absolutely not yet ready for it.
Divinity: Original Sin provides a “gateway battle” to prove it. If you wander over toward the church, you'll encounter a Charred-Bone Archer who is at Level 8. It's possible to beat that single enemy even with an underleveled party, but if you venture further, you'll find that you'll often be outnumbered by additional Charred-Bone Archers (and worse) that are Level 8 or higher."
I assume this is your problem. The guide now says
"That means we need to spend some time increasing our levels, as well as bettering our gear. The best way to accomplish that is to get some side quest work done.
The first thing we should do is at least trigger the quests for the church. Not only are they along our way, but you can kill that one Charred-Bone Archer to get a taste of what's to come, if you want to test your strength. Go to the Legion Headquarters at the east end of town, then go out the north gate beside it to the left. Check out the screenshot below if you're not sure what gate we're talking about."
It then gives you tips on other side quests available to level your characters to 6.
Part 4 is instructions to reach level 8. At this point, the guide simply says you should customise your team with better gear. Since this is still divinity, I recommend *another* guide for how to build your characters.
Part 5 will lead you to the church where you finally may begin to wrap up the investigate the murderer quest. Just play, but if you get stuck, don't feel bad about researching.