This isn't going to be a level by level guide because from my perspective they pretty much always played the same way, so here are a few tips that helped me get through the levels.
-Be mindful of how far your max weapon reach is. When you get the A-wing/Interceptor missions they'll let you edit your loadout. I would always use the longer reach primary weapons because getting up close usually is almost certain death.
-Use your hull repair and supply request if you're at 50% health. This helped me at least manage the cooldowns of your repairs and can keep you alive longer. This is especially important when playing as the empire. Also, every checkpoint when you die, you get full health and your supply request is charged.
-When you play as the New Republic, when you put power to your shields and charge them up, you can switch to weapons or engines and your shields will stay overcharged for a good amount of time. When you take power from the shields the overcharge goes down but it's pretty slow. Also get used to managing the position of your shields. For instance if you lose your primary shield but you still have overcharged rear shields, balancing your shields will refill your primary forward shields. This goes a long way, especially late when you're constantly fighting corvettes and raiders.
-After mission 3, where you're defending the star destroyer from Imperial bombers at the beginning, you get access to the squad command prompt (Y). Use this all the time, and if you have are being fired upon and double tap "A" you'll target the enemy firing or tailing you and you can command your squad to attach them. The squad is very good at handling targets, and if you can take advantage of them being invincible to send them after corvettes and frigates while you maintain a safe distance dealing with fighters. This is essential when playing missions where you're forced into the support ships, because you can't really destroy fighters, as you're usually forced to use the ion cannons.
-Probably the biggest thing is don't stress about medals and optional objectives. They aren't usually worth it and getting to the next checkpoint should be a top priority. If you're going for medals and optional objective achievements I would do those on story difficutly.
I found imperial missions way harder than the New Republic because of the lack of shields. So knowing where enemy fighters are and how they are going to aggro on you makes a huge difference. That just comes with practice and experimenting. Most of this is trial and error like most games. I didn't mess with the shields, engine, or hull when doing loadouts because you need maneuverability and shield recharge to survive. I did use seeker mines when given the opportunity but to be honest it didn't help more or less than having missiles did. If you have any questions about a specific level please comment and I can give a general idea of what I did to manage.