Put all of your training camps to use at all times, as often as possible. Once you activate them, extend them out as far as you are able, so that you can train 4 or 5 survivors at a time. You need to have standby survivors to do this, so if you are running out, go to the main missions and find more survivors. Once your houses get full, then put them in the training camps to clear out your houses.
All of these survivors can then be used for extremely fast leveling up, since two-star survivors work a whole hell of a lot faster than one-star survivors for these purposes. Level up your main guys of each element (should be three, four or five star survivors) until they hit the level where they can then be upgraded. If you have the items necessary for the upgrade, then do it (if not then they can typically be found on the world map).
After level 5 you’ll unlock the roadmap which will allow you to play special quests. These quests are more difficult than the ones on the world map, but typically they will give you higher rewards for beating them as well. Make sure that your characters are levelled up at the very minimum (if not also upgraded) before you try to play these quests. Even the ones marked “easy” tend to be pretty difficult.
Auto mode works well enough in battle for most of the quests. Typically later in the game, the quests will get tougher, especially the last segment of a particular level (these tend to get a lot tougher). When these come around, you’ll need all of the strategic advantage you can get, and the computer doesn’t always match types correctly. That’s when you’ll need to take over and do the battle yourself, especially when item use comes into the equation.
The choices that you make during the dialogue sequences have only a minor effect on the game, generally. The biggest effect is what kind of rewards you win as you go through the game. Take one side, and you will get different rewards than if you take other sides. The story will also branch off a little bit differently, depending on the side that you take.
Don’t sacrifice your three, four or five star characters even if they become redundant. The reason for this is that once you have more than 5 rares, now you can mix and match them depending on their type/element for a huge advantage. For example, if you lose a battle and you notice that they are all of the blue type, you can then go heavy on characters using the red type in order to counter them more effectively.