While S&S4 certainly has a lot of problems, "Mario Party with permanent character progression" is a very unique type of game that can be fun to farm sometimes.

Random events, much like in S&S 2 (where you can farm them by repeatedly entering and leaving the blacksmith shop), are THE way to get absurdly overpowered in S&S4. You can get more stats from them than from leveling. Stat gains from adventures are permanent, so I prioritize them even over winning the current round. So, the goal is to maximize the amount of random events per XP level we get. AI players scale to your level, not to your stats after all.

The "Yes/No+ columns" are the positive outcome for which your chance is listed ingame, "Yes/No-" are the negative outcome. A "-" outcome means nothing important happens, a blank space means untested.
A reason some negative outcomes are untested is that it's best to use only the "Fate's Smile" card from level 20 when you unlock it, until you have 150 or so Charisma and the chance of getting the good results is extremely high anyway. Then you can swap to the "Helmguards Betrayal" card to slow your opponents down to a crawl while you farm stat events.
Don't start dumping points into Charisma until you're already extremely strong, unless you're a bard. The card swap is a big deal once you're able to do it though, as your farming run ends and you get "punished" with XP when anyone makes it to the finish line and you're stopping a lot on purpose.

I've found the best map to farm stat events on is Archmagius Mountains. If you'd move past a random event tile, but are able to stop your movement with a Minigame Castle or Duel, always stop. In the first of the map, there's also a "reverse direction" tile soon after TWO random event ones, stop yourself if you'd move over that too. I prefer playing against two AIs (with only one, the game would end if a single duel to the death happens). Three AIs are probably even better for maximizing stats especially when you're already using Helmguards Betrayal, but it slows each round down a lot too.

The table will give recommendations for the Mage class because I found that's the strongest class, after enough stat grinding you can simply oneshot most opponents. Put most stat points in intelligence, the rest in vitality. Prioritize buying strong damage spells.

Oh and one last tip: When you get the Animal Race Minigame, bet on the lion to win. In my experience, it wins something like half of the races.