Poss Spoilers******Invincible********

The best thing to do is to target the bombers and hunt them down as long as they don't shoot torpedoes. When they begin to fire torps, you just have to act quickly by intercepting them, and after that, destroying the remaining bombers.
It's not easy to kill a bomber wing on the higher difficulty levels since they have more armor then, so try to lower the difficulty a bit if you can't win this mission after your retries...
A last thought: If you don't let the bombers come too close to the Invincible, her laser turrets should destroy a few torpedoes even if you didn't manage to intercept them.

By the way, if some Kilrathi ships are invincible, that is not a bug, but a by-design decision - In some missions the scripts and gameplay only work when there is a Kilrathi carrier, so destroying it would break the mission flow.