You need 3 or 4 different teams, each with its own very separate play styles to get through. The different teams should be roughly the same gear tiers, though probably you'll have one slightly ahead of the others, then the others all more or less tied.
One of the very best squads for GW is Jedi under the leadership of Bastila Shan since she grants a huge amount of bonus protection at the beginning of a fight, and no matter how many times the enemy burns through it, it starts at maximum again (though if you lost health or regular protection, that does not return to maximum. Fortunately, though, Jedi have a lot of healers to help get your health back up).
But some enemies are just very good at killing Jedi, and others ignore protection (Dooku being the one you'll probably see most often.) So you need to have different teams to exploit enemy weaknesses and to prevent them from exploiting your own.
It's a good idea to keep most of your toons levelled to less than the maximum. They usually should trail your character level by 3-4 for the top 2 or 3 characters on a squad and by 3-6 for the remaining characters in a squad. (Outside of the 3 or 4 most useful teams you have for GW, other toons should be levelled even less.)
Also your mods make a huge difference here because they add only a small amount to the character power calculation, but they can add a huge amount to your fighting ability. Work on those mods!
Finally, once you have your 3-4 decent squads, make sure that you give each of them a chance to fight one of the first 4 fights so that they can pre-load turn meter for the next time you use them. Make sure you pick a fight with an enemy that they're well suited to defeating, and if you lose a character or even just get a character too damaged, back out and try again (possibly changing teams, if you think you need to).
From levels 65-82 GW is at its worst, but it does get better eventually.