Feel free to add in your own point and views to help other players.
Buying potions and manaEdit
A lot of time through the game, and you'll start to notice how overpowered the enemies are. There can be times of frustration and annoyance to get passed an enemy and to continue through the game. Which is why it's important to buy a lot of potions and mana. An insuffiecent amount/being unable to predict how many you need can be disasterous, espically if you don't have enough gems to cover for it.
How to earn money and gain levels fasterEditSimply: battle more. Or to put it another way, go through training. At the begining, you will be guided to a glowing blue stone-which not only gives you HP when you're running out of it, but also helps you train. The more you train, the more progress you get. You can earn, EXP, money and your health returns- if you're low on it-when you start the battle. Of course, unlocking more training gives better advantage in money-however it can still be overpowered. A suggestion would be is see how far your limit is. Once you know your loss, you can go to the easier levels to work off some EXP or money. Or if that takes too long, you can look for missions by local people. Though technically, they make you travel far distances so it may not always be as effective.
Effective use of GemsEdit
Gems-which are used when you capture a demon-are more or less rare to find, so when you get one-you should hold onto it. I personally have never had to buy gems-and neither would i advice it because it costs real money, not game money. You can find gems by looking out for treasure chests-they are usually on a secluded part of the map(s) so work on looking for them. The more you save, the better. And another thing: the more you want to save up-the more you should watch out for capturing demons. Most of the demons first caught are weaklings. There are a few missions and key events which give out free demons. Part of it will be using your own judgement. As a suggestion, wait until you come across harder demons, they should be more reliable. Don't waste gems on getting demons to forge them together-they will remain weak and only their skills will increase. So unless you have a large amount of gems you're not afraid of wasting then you should do so.
There may be points in the game which lead to a game over. And you get given a chance to use gems. I would suggest, not using it up until you literally tried and tried and are left frustrated. You could have a lucky chance, and if you know your luck has run out, then that's when you should try looking for alternatives. You can buy revive scrolls in normal circustances-so they are either unlocked by doing something in the main event (like beating a boss) or by a game over and buying a scroll using a gem. Though it comes down to your own choice how to do this. I only advise this as I've already completed the game by the odds. It really is time-consuming...so it's better to be prepared than irritated through endless gaming.
Choosing demonsEditLike said before: the first demons you come across will be weak. Don't waste your time capturing those. If you recall the begining of the game-which Iron had some demons-try and aim for those type of demons. Or, if you are more of an explorer, a suggestion would be is look out for what type of skills an enemy has when you come across it. Plus, its type. You should vary your types from: fire, water and earth so when you come across a certain elemental boss, you will have an even amount of chance.
Also to note out: although you can only use to about four demons with different abilities, you can change your demons at any time. Even in boss-fights. However, you will have to wait for it to load before it comes useful. So be prepared!