To be honest bro. I beat it nearly every time and I haven't spent a dime on the game. Generally the last opponent is 5-10 levels higher than me. Maybe even more sometimes but I don't pay attention to that. To go farthest in the galaxy mode make sure to only use your special moves if you can have them fully restored by the beginning of the next battle. The only special move you want to use at the end is a healing move. Level up Ewok Elder and have him with you as much as possible to revive your characters before the end of matches and remove all status effects. Keep Chewbaca as much as possible to act as a punching bag when other characters are low on health and waiting to be healed. Attack the other teams healers first one at a time so that they can't restore health and slow your progress. If they have Darth Sidious get rid of him before the healers because when he attacks he can cause your characters not to be able to be healed. After the healers, get rid of the other players one by one starting with the lowest health meter to the highest because you can get rid of them quickly and lessen the amount of opponents attaching you each turn. If you do this and also add more strategy based on what you learn, you may not win every time but you will win more times than not.
If you are still having problems you made need to toughen up your core team. Have a core group of 5 characters that you have stocked with mods, at as high a level as you can get them, with gear upgraded until you can't add gear due to your level and with your attacks upgraded in a way to function as well as possible with the way you play.
Your core team needs to have one character like Chewbaca who has high health and can take a beating, the Ewak Elder to remove status effects and revive characters, a healer like Jedi Consular or Luminar Unduli, and two characters with strong attack specials like Luke Skywalker, Genosian Soldier, Qui Gon, etc. Only use one healer at a time so if you lose you can have another for the next battle if you have unlocked them. Try to get your main group to a high star count also. It makes a huge difference.
Each day you play, once you have upgraded your core group as much as you know how, use the rest of what you have to upgrade the next chosen group of 5 characters. They won't be as strong as your core group, but many times when I get to the last battle, I will only be able to get rid of 2 or 3 of their guys before I am wiped out. Then my second group is strong enough to finish the job as the opponent will be outnumbered.
Keep it as simple as it needs to be for you. Build up your core group and fight the way I shared with you and your odds to win will be much better. I have had periods of time where I couldn't beat it also for a few days but never more. Shortly after I'm back on a winning streak as long as my losing streak or more.
