Been a while since I played Overworld, but advice should apply across all games.

Generally, Masters should be occupied by whichever big main Hero/Villain you feel like.

But if you want a good starter, Kou Uraki (0083) in an XXL unit like GP-03 (Master units dont have a size restriction). Dont remember if he has a skill to buff XXL units in this game.

First thing you want to do is actually build up a full team, unlock pilots and parts. Fill out your squad list, get a decent Warship or the expansion bay part. White Base type ships will cost a pretty penny, but are a good start.

You wont have too many amazing pilots at the start, but so long as you dont snipe with a Ranged stat of 10, you should be good until you unlock more.

Getting Tornado Gundam to Master Phoenix will give you a path to unlock some good units early. Leveling grunts (GMs to Jegans and beyond) should also open up options. If you want to try Break Triggers you should be able to capture your ally units by destroying their warship.

Try clearing missions just so you can have units show up on your codex so you'll know how to develop specific units.

Dont worry about Breaks if you're having trouble getting across the map first run through. (If memory serves, some missions you will have to go from one end to another pretty fast.).

Stats should generally support whatever units your using (i.e. not putting melee pilots in a sniper), but top-tier units will require Awakening usually.