Are you using CC? Unless you're running Guardian/Shadow/Vanguard/NotASmuggler or the Empire equivalents, your group should have at least one method of keeping an enemy out of a fight for a minute at a time.
Commandos can Concussive Round anything.
Sages can Force Lift anything.
Sentinels can Disable Droids.
Smugglers can Slice Droids.
Scoundrels can Tranquilize non-Droids.
You can't CC most elite mobs, some bosses are even immune to taunt.