Doing as many Galactic War Nodes as you can each day gives you many worthwhile rewards. It is your best stream of consistent income.
I think, the best way to win Galactic War is to have several teams/squads of characters at comparable levels. The way the computer picks which teams you will face in each node may seems random, but it follows a predictable pattern. Generally, the nodes get harder as you go, with the purple nodes being the hardest of each set of three.
If a battle is going badly, you can retreat. When you enter that battle again, you are reset to the same exact state. So, if you attack the same characters in the same order with the same attacks / abilities, you will get the same result. This allows you to try different approaches to find the one that generates the best result.
Any unhealed damage and the state of the cool downs for your team carry over into the next node. If an enemy team defeats you, their damage and cool downs carry over into your next attempt to beat them.
If your best team can not defeat a node, then try retreating and using your 'B' team. If the 'B' team can weaken your opponents even just a little bit, then your 'A' team may have a much easier time winning.