Out of curiosity, what's the Power difference between your first character and, let's say, your 15th? Gear difference?
Galactic War teams are based on a percentage of your five highest Power characters, whether you use them or not. In most cases that means your Arena team, since the intuitive thing to do is focus investment on those five characters. But this makes your GW harder, because node 12 is something like 20% more power than your highest characters. (I don't remember the exact amount, so don't fight me on that--but the idea is correct) So if by your final node you're down to two teams, you're going to be out-powered significantly.
The best thing you can do is invest in three or even four teams. This sounds onerous as hell, but I'm not saying each one has to have full mods, gear 11. I do the bulk of my GW with a team of Ackbar, Leia, Lando, Consular, and Chewbacca, all gear 8 (Leia and Lando are 9) and with basic mods (ie: they started white and don't have optimal primaries or secondaries--those go to the 1st and 2nd teams. When I get more I'll add them here eventually).
Use each team once in the first few nodes. Then use whatever team to do the bulk of the War--BUT NOT YOUR BEST TEAM. Only use your best teams on nodes 6, 9, 11, and 12. This preserves their protection, cooldowns, and turn meter advantages for when you really need it: the last two nodes.