As with many RPG games in the third-generation of consoles, this game can be unforgiving and difficult. There is also a lot of luck involved. Thankfully, you can save at any point during the game. It is highly recommended to save before doing any critical moves such as before attacking a boss for the first time. Otherwise, you could place yourself in an improbable or even unwinnable state and will have to restart the whole scenario. Keep in mind that, while you can have up to three save files, you can only have one save for the middle of a level. Use it wisely.
It is also recommended to go through the Characters page as not every pilot will remain in your party. Remember to prioritize leveling up units that will stay to the end of the game first before improving your temporary help. Regardless, the level guides will notify when someone is leaving the roster for good. On that note, there's no permadeath in the game. Destroyed robots are repaired between levels as long as you have the cash. The more upgraded the unit, the more expensive it is to repair.
Remember that there are many elements in the later games that are not present here. The most important difference is that leveling up is the only reliable way to improve the maximum health and accuracy of your units. (The items either improve too little for the price or, in the case of accuracy, only temporary.) Make sure no one is left behind too far as you progress through the game, or it may become nearly impossible to level them up without Spirits or expensive items.
Finally, most levels do not have a turn limit. Take advantage of this to give your units optimal positioning. Good luck, commander.