Honestly, even with many bugs, the game is still playable, and as someone who had to search for help up here just to get past the stupid maze in the cemetery, here's some guide:
If you hate the prince and made it your job to beat him every time, it's possible to beat him even on first playthrough. That focker's shield takes on all damage even if you use an AoE attack. Luckily, just beating him is enough so after blasting through the shield, you can dump all manner of damage on him. A minimum of lvl15 and a fully upgraded straight flame sword is plenty of leeway for this fight. That said, bring bombs for the sister to use too. About 5 or 6 would be enough. The flame sword is plenty good but the main good point is that you can craft it even before finishing the ruins.
Next, about the gun quest, finish the lost horn quest and hunt the unicorn quest. Give the guy the horn and wait for a day. Then he would have a stall, and would give you a flintlock and three bullets. Go out and fight three enemies, use those bullets in battle and then go back. That said, make sure after every battle, MC talks about the gun. Only after seeing it three times can you finish that quest. After that you'd get the gun mod recipe, which can be crafted in MC's home.
Edit 1: in v1.09, if you somehow fire all three bullets without triggering MC's 3 dialouges, the quest will be bugged and impossible to clear. And I don't know how it happened either.