In general, the best way to attack the Rancor/Gamorian Guards is to limit the number of times they can attack. Since stun and ability block don't work on the bosses, any character that reduces the boss' turn meter or slows them down is very useful. Teebo and Vader (with zeta) are the two top turn meter reducers in raids.
If you have a bad run with a team (like Teebo gets eaten on the first turn), then you can always retreat and try again. Unlike Galactic War, the random number generator will produce different results.
This is a hard phase to start with. When you start, you will be spending 90% of your time killing the side guards so that you can get to the main guard. But after a few months, you will be spending only 10% of your time on the side guards and doing large amount of damage to the main guard.
Every, semi-random number of turns, the Rancor will eat someone. This attack is unavoidable. To give you a fighting chance, if you attack the door panel on the side then the Rancor will be immobilized for awhile. Your attacks will do more damage, but you can not reduce the boss' turn meter.
Similar to Phase Two.
Similar to Phase Three, but no door and no devour.