Is there a way to increase monster encounter? I want to raise party level so bad and the only one I usually got is when they go to a corner or a room-like area
Raising party level is usually only useful for the lv100 trophy or to get a lv10 thief for a Free Mode quest.
The most efficient way to strengthen your party is by knowing how to use item and getting good equipment.
For example, in the early way you can spam Tomahawk (use item midcombat and double click it). It kills a row of everything weaker than a harpy. Two Tomahawks kills harpies.
After the early game, you get good equipment very quickly. For weapons, focus on hit counts and give every frontliner 2 weapons (first weapon is a Main and the second weapon is a dual).
Baclline fighters usually can't double wield, the exception are Thieves and Assassins who can dual wield Boomerangs in the early game and Chakrams in the early game.
Assassins and Monks can fight barehanded and it counts as using double weapons, although to make the most with them, they need to have a weight under 5.
Magic is very weak in the early game and serves more as support than damage, like using Slience, Sleep or Blizzard on a big row. They usually hit harder doing physical attacks equipping a long staff like Holy Rod or Silence Rod.
Every wand except the wooden wand can be used as an item too.
Healing wand does line heal (Multi Heal)
Fire wand does line fire (Fire ring)
Ice wand dows line ice (Blizzard).
Thunder wand does single target lightning (Thunder).
Skull wand does single target death (cursed equipment).
Star wand does a powerful full aoe non elemental attack (Star Burst, the strongest Alchemist spell, only endgame monsters survive this). This is an endgame equipment, you can technically find it in the early game, but the hint comes from a lategame optional dungeon.
Using a weapon as an item a number of times breaks it and the repair price depends on the weapon tier so don't use the expensive ones against trash.