Exp+Gold and Item gain: Edit
To farm gold or EXP: Edit
Gold and exp will always be calculated based on the number shown on your screen. If you want to maximize this you can go to a lower area where those values increase and farm there. You will need to find a balance between speed of killing and exp given per fight in order to maximize this.
To get Drops: Edit
EXP and gold are calculated based on the numbers shown on your screen, but item drops are not. To get the best drops you need to have the highest kill speed possible (5 members one hitting enemies) and then transfer to the hardest area you can enter and either close the game to AFK, or use diamonds to speed up time before a fight finishes. The time spent afk or skipped with diamonds will count as you killing the high ranking area monsters at the speed of the previous fights.
Combine both of the above tips to get the best of both worlds: Find the highest area that you are capable of one hitting everything in, wait until your exp/gold values are stable, and then head to the highest area available and afk or use diamonds there. This way you will get about 70% of your maximum EXP and gold, while also about 10x the item drops.
Boss fights: Edit
Many bosses have setups which are easily exploitable. The simplest formation to exploit is one where the enemies spawn on the bottom two rows. If you face a boss setup like this, you can simply counter by putting your characters all on the top two rows, preferably in a way that the damage is split evenly between the group (3v3 and 2v2). In this way, the enemy is dealing essentially half damage to you while you are focusing 100% of your damage on single enemies at a time, quickly removing them from battle.
When rebirthing it is important to consider what factors affect your progress. You should weigh the pros and cons of what you invest into if you want to progress at a steady rate. Do you progress slowly at places because of levels? Gear? The need for new shop items or party members? These questions will help you choose your path.
Experience needed to level up is likely an exponential slope of some sort, yet your investment in experience levels increase rate grows negatively linearly. These two factors combined mean that your experience gained through rebirth point allotment gives largely diminishing returns. With this being said, leveling up is one of the fastest ways to gain strength in the early game, so adding points in experience can greatly speed up this part of the game despite its slightly lesser impact later on. Leveling up is also the most important aspect of Adel, your starting character. He gains huge stat boosts at level 220, which improve even further at 300 and above. But, investing into this with that goal would be futile if you won't ever reach a point where he will surpass other two star characters.
Money is something which is almost always a necessity. While it is not necessary to buy new gear from every town, or even new characters, many towns sell gear that is much stronger than is currently available through drops. You will likely always be short of money when you play, but the question to ask yourself is "will putting points into this stat allow me to access more items or characters before I expect to restart?"
Item drop rate is the hardest of the three stats to increase through your gear. Items may only spawn with up to 12% item drop rate increases, while you can find items with 30% EXP and gold drop rate increases. With this said, as far as gear efficiency is concerned drop rate is #1. Items later in the game begin to get stats that dwarf your character's base stats, while earlier in the game the items found are weak compared to your character. Item drop rate is one of the only ways to find more items, and just like money, will always be important.
Diamond Farming Edit
It is possible to gain a diamond every 15 minutes on the dot, though there seems to be a limit for how many you can receive in a day (20-30). If you haven't played all day and start at 6-8 pm, you can essentially open the client and click the bonus button (the video icon to watch an ad for one diamond), set a timer of 15 minutes the second you accept, and then open the game when the timer goes off and have a new bonus available, repeating until midnight or later.
Just be sure that you complete one or two fights so that you don't encounter the gold+exp loss bug.