Every game is basically the exact same, with the minor difference of accomplishing some simple task that you'd likely almost complete anyways. The real way you win this game is by getting four heroes at lvl 40 (or high 30s), and near perfect gear. In other words, the two things you need to do to win are max blacksmithing, and level up.

Starting out is the hardest part, because you suck and have nothing. Here's some steps to get you into the game and ready to go.
1. Always pick blacksmith, regardless of task, because levelling blacksmithing will ALWAYS be more of a pain than anything else.
2. Re-roll stats for good hp and strength.
3. Recruit someone from town.
4. Search for treasure in a field (easiest place because it's open), and while doing so, fight the mobs to level up a few times.
5. At this point you should have enough raw materials to make some basic armor to make your two people somewhat resilient.
6. Take over a couple of bronze mines near the starting location.
7. Find a river, and travel down it until you're in the level 40 area.
8. Find a town, ditch your follower, and recruit someone new (who will be level 40).

PROBLEM = If no one will follow you, craft all the potions and daggers you can, and go fishing a ton to level up cooking. This will hopefully make at least one person willing to follow you. If not, go find a new level 40 town and try everyone there.

JUST IN CASE = I played half the game before realizing how to use combos. Press up or down with attack selected, and it changes what combo you will do.

At this point you have a level 40 follower, which will allow you to power level yourself.
1. Travel back to around the level 10-20s area.
2. Take some mines, at least one of each type if you can.
3. While doing this, you will occasionally get a battle where your character will survive, and your lvl 40 will do all the work, which will seriously boost your character's level.
4. Keep taking the lower half tier mines, while boosting your level.
5. Once you have a few mines, and your character is about level 20 or so, return down the river to the level 40 town again.
6. Craft everything you have, going up tiers in smithing (bronze, iron, steel) and cook a bunch to boost your blacksmithing/alchemy/cooking levels.
7. Recruit level 40 characters in town so you have three followers. Try to get followers with high strength (80-100), and high hp (8k-10k).
8. Now that you have a super party, take some mines in the 30s area, which will once again boost your characters level substantially.
9. Make sure you get a lilithium mine or two.

You should now have some mines of every sort, and your main character should be in the 30s.
1. Go to a lower level area, and search for treasure. Treasure is actually the fastest way to gather wood and leather, which you need lots of.
2. Just keep finding treasure chests (the enemies will avoid you because you're way stronger than them).
3. The program "Cheat Engine" can boost the speed of your game by about 3x, which will make the grind way more tolerable (if you disagree with doing this, totally your call. I don't find it cheating because it's not making the game easier, it's just making it take less time to walk because your character is painfully slow).
4. Craft until you get to smithing level 26.
5. Keep spam crafting lilithium daggers until you reach lvl 30 smithing.
6. Craft all weapons and armor for your entire party.
7. If you run out of leather and wood, hunt more treasures. If you run out of lilithium, just travel by sea a bunch to waste time so your mines will get more.

You should now have a party of all level 40s (your main should at least be high 30s), and you should have all perfect lilithium gear for all members. This means you can easily defeat the boss. An optional step would be to ditch followers that are weak (even though they're lvl40), and switch them with new lvl 40 people that are high in hp (like 9-10k), str (90-100), and who have a decent weapon (basically just not the mace, I don't know why SOO many people use the mace, and it's the worst weapon).

The tasks are incredibly easy and straightforward, and actually quite pointless.
Doing the steps above, you should have maxed smithing and your main level already.
To get rich, sell all the lilithium daggers you made. If you're short, make a few more.

The only real problem is the leviathan = Catch 5 of each fish. Remember that you can also fish in non-sea tiles. Just change your weapon from the bow to the rod, and look in the water. Once you get 5 of each, go back and forth between plains tiles until you get the prompt.

Now fight the boss, and easily assure victory.