Eventually, missions get quite tough to complete if you want an A or S rank.
An easy way to avoid taking extreme damage is to use this booster trick.
Get familiar with this first: tap forward twice (or left twice, or right twice) so you pull and evasive dash. Keep the button depressed when you press it the second time, and then tap X to boost. (Release the button) This will cause your mobile suit to do a ranged 'skip'. Before you land, you can stack up these skips to travel pretty far, pretty fast. Even though your booster guage might overheat and run dry, so long as a tiny sliver recharges, you can skip again.
You can use this to perform massive aerial strafes around targets, and bombard them with fire from the sky. This works especially well with any mobile suit that can fire missiles or homing rockets/torpedos, since the wide strafing arc in the sky can send these projectiles homing in at the same target, but from different trajectories, increasing the hit rate of these weapons.
Pilot stats level up according to your actual playing preference. If you use melee attacks a lot, for example, your melee attack power, homing ability, and range, will level up. (They all fit into one basic melee category)
If you evade a lot, your evasion efficiency goes up, etc. It may take a few battles to rack up enough experience points for you to get +1 in that stat field, but getting high ranks in missions speeds up the process.
There isn't a stopping point for these skills, in terms of only being able to get so many points in a few amount of categories. You can get your polygonal stat chart to max out on all ends simply by enacting their triggers in battle. IE: If you don't really play using close combat weapons, you can still always go back into battles in free mission mode, and level up that stat later.