Quick tips for You about engines I prefer to use at all stages of my mission to Duna. Of course quantity and types of engines and tons of fuel depends on mass You want to orbit and deliver to Duna orbit or its surface: 1.Going to orbit: Rockomax "Mainsail" for sure... or LV-T45 for smaller crafts. If more thrust needed give some more mainsails every with 3 or 4 Rockomax X200-32 Fuel tanks 2.Inserting parking orbit: Rockomax "Skipper" or Rockomax "Poodle" or LV-T45 for smaller crafts, You need some thrust to insert orbit fast. 3.Interplanetary transfer, orbit changes, nodes correction : there You can use more efficiency engines like LV-N Atomic Rocket Engine, add 3 or more if You got heavy craft. Remember they are efficient but very heavy! You can also use Rockomax "Poodle" or LV-909. 4.Braking to establish orbit on Duna: master aerobraking but if developers end their project it will be harder because of heating. Else, use the same engines like in point 4. 5.Landing/Descent: I preffer radial engines:Rockomax Mark 55 or Rockomax 24-77 or new one: LV-1R Liquid Fuel Engine. Other small engines are also good. Dont use LV-N Atomic to land on Duna because it is heavy and loose some efficient at surface. 6.Ascent: It isnt hard to get in orbit on Duna. Remember to take some fuel with you. But like dlmartin wrote You need to practice orbital/landing techniques. It is more important than engines used in any phase of mission. Gravitational assist is also useful. Have a nice day with creating new duna mission!