Light armor for avoiding detection is best. At the higher difficulties you want to avoid as many fights as possible because when the flights do happen they are nearly overwhelming.
So you only want to engage when it’s on your terms and engagements should be fast and brutal to avoid any bug holes forming.
Good scouting is essential. You don’t want to blunder into a patrol and if you see one skirting your position avoid it if possible.
Planning your route is also very important and you want to optimize it so you’re not doubling back whenever it can be avoided.