Environmental objects like bushes can't be hidden in, but can block line of sight. Use them to your advantage against ranged enemies, and destroy them to enable ranged allies.
If you're having trouble with a particular mission or just want to maximize your party's effectiveness, you can replay old missions. Doing so will still earn you XP, just less each time you replay it (to a maximum of -90%). You can also earn gold, just only by defeating enemies (which spawn it randomly), and not by completing optional objectives. Gems cannot be acquired at all through replays.
Focusing bosses before mobs is rarely if ever a good idea; take out the latter first to stand a chance against a boss.
If you've upgraded the Guard ability to grant armor to allies, move the allies near Leonhard before using Guard.
Use the Bandage ability to heal a little whenever you aren't under significant pressure and another ability wouldn't help you more. Over the course of a mission, the extra health will add up and make things significantly easier.
Chests can be destroyed, preventing you from acquiring the treasure within and from completing their associated optional objective. This includes with abilities like Charge.
Party members earn full XP whether they are alive at the end of the mission or not.
Generally speaking, the more rewards a mission has, the more difficult it is. Do easier missions first.
After you complete The Graves of Brothers Grim, buy and upgrade the Willow Flute as soon as possible as it grants you three more party members, thus making missions significantly easier.
