Tips:
When you begin getting more characters try diversifying your roster as well as increasing supports amongst your characters. This will help in forging/performing longer chain attacks (A concept that you will soon encounter). Supports can be increased by performing Chain Attacks, helping with quests, recovering characters from topple/daze, etc.
Always try to cheer up your characters during battle, you can examine their expressions on the upper left hand corner of the screen. Depressed/moody portraits signifies decreased hit/critical rates or it will indicate that a character is toppled/dazed, this is your chance to encourage your ally or help them recover from topple/daze. This will increase support as well as bring them back to normal fighting conditions.
Having someone divert aggro onto themselves such as Reyn is always helpful, especially in the early levels. While Reyn is an aggro tank you will eventually encounter someone who is also good at diverting aggro but utilizes Agility instead. (So an aggro dodger)
Selling off your collectables is a good way to make some quick cash if needed, however, you can also gift them between party members and increase support. There are certain points of no return in the game which can greatly limit the unique collectables from that area and the only other way to obtain these specific collectables is by trading very specific people. ****There is a large side-quest that I highly recommend**** and it will require you to collect items from all over the game in order to complete it and some of them are located in areas of no return.
You can increase trading options within certain regions by increasing your fame/popularity within said region. This can be accomplished by doing quests within a particular region.
Crafting gems/cylinders will be crucial if attempting to tackle the strongest monsters in the game, especially Night Vision gems. (The highest tier is level VI which can only be achieved by crafting). Other good gems to have are Agility+, Topple+, Double Attack+, etc.
Pay attention to your skill tree. If you max out a skill it won't automatically switch to a different branch, ignoring this for too long will waste consider amounts of SP points that you accumulate. You can also unlock additional branches for each character but that will require many quests to be completed.
If you are not playing as Shulk remove the skill Battle Soul, its usefulness is extremely niche and will usually hurt more than it helps.
By the time you get skills to level 5-7 you should figure out which ones you will be constantly using, which ones you don't find very useful, and start maxing out your 'best' skills. The AP costs increase exponentially as you level up skills, therefore experimenting early should cost very little AP comparatively so don't sweat it.
When scanning monsters you may notice that some have a more detailed border as well as an adjective behind their name/ a unique name. These are unique monsters and are considerably more stronger than their normal counterparts (You may even have difficulty killing a unique monter that is a lower level than you). However the rewards for defeating them are much greater.
There are no items you can use in battle. Hence this is why you recover incredibly fast outside of battle.
You can usually take on monsters that are up to five levels higher than you (though it will be a difficult task) but lowering the level differences between you and the monster will prove to be incredibly helpful. Killing a monster with a level difference of two is much easier than killing the same monster with a level difference of five.
If I can think of anything else I will let you know, other than that I hope you enjoy the game!