You'd certainly need something beyond a chemical rocket but there are options. At that density a turborocket[1] would certainly be worth the extra mass but I think still wouldn't be enough of an advantage. In the realm of roughly existing technology, a nuclear ramjet[2] could get you to the upper reaches of the atmosphere and give you a nice little initial boost as well to your speed. And in the realm of SF, a nuclear saltwater rocket[3] would still be easily capable of making it out of Venus in one stage.
To get a turborocket to work in an atmosphere without oxygen you just have a classic rocket engine, with its own fuel and oxidizer, use its exhaust to drive a turbine the same way a jet is used to drive a turbojet. You have to leave off the afterburner stage that many existing turborockets have where you inject more fuel to burn after the final turbine.
If you're using air for just working mass and not for an oxidizer, it's my belief that this does not improve specific impulse but only improves peak thrust/engine weight.
Because the energy lost to the movement of the propellant goes up as the square of the velocity but the thrust goes up linearly. Bulking out your propellant by a factor of ten while reducing the exhaust velocity by a factor of ten doesn't change the resulting thrust, but reduces the energy you need for that thrust by a factor of 10.
