Which are the best job combos for each character?
There is no "best" combination for characters the game is very accommodating to your play style. Certain bosses require tweaking of your favorite set up but it's really up to you.
Whats that strategy involving knights/merchants to cap PG overnight?
You just need a couple of things.
1. Someone with Merchant at lvl 11 or higher for Big Pharma or just ablink to someone who has it mastered. This ability heals the mob for an amount based on your MND stat and gives back PG equal to the amount healed. Best to have Agnes or Tiz be your Merchant due to their high base MND.
2. A Blessed Shield user. Someone to use the shield for free Cura to the party to heal up any damage you will take while you sleep. Again someone with high MND here and be a Salve-Maker with Holy One ability to maximize the Cura.
3. Someone to deal damage to the enemy but not more than what Big Pharma cures. What you want to do is deal less than or equal to the damage Big Pharma cures so the mob will stay at max HP forever.
4. Just have 1 enemy that doesn't do any ailments to the party. A single Raffleselia work great.