For Rancor raids, you need some good Turn Meter Reduction (TMR) characters. The best ones you should shoot for are Teebo, QGJ, and Rex. It's also good to have either Tie Fighter Pilot or Dengar to put a Tenacity Down debuff on the rancor.
For the arena problem, perhaps try an Emperor-lead team.
EP / Vader / Biggs/ Wedge / then for the final person switch it as needed. Royal guard if you are dying too quickly. Nihilus if you need a dispeller. Maul if you're fighting a jedi team.
Also, you should level up your arena team, all 5 of them, to your maximum level possible at all times. If someone sees your arena team with level 70 characters, it's a sign that your team is vulnerable, or your roster is too spread out. Focus leveling your chosen 5 to max level at all times and you will find it's much easier to hold your arena rank.
And if you need help getting credits for leveling characters, I cannot stress enough how helpful Lando is in the credit heist.
Good luck!