The VST plugin SideKick (about $30 from BeatRig) does a fantastic job of sidechaining in Cubase Elements 8. Surprisingly easy to use.
Create two tracks, a kick drum and say a synth. Add Sidekick as an insert on both your Drums (trigger) and your Synth. For the trigger track, leave the power of the vst plugin off but notice the bus number it shows. Note that this power button is part of the vst itself and is not the power button to disable that vst.
On the copy of Sidekick that is on your Synth or Strings simply click the number on the left side that corresponds to the number that was shown in the trigger copy. See screenshot here:
So the trigger is configured on the right side, the actual audio (sorry term escapes me) is on the left in my screenshot, in this case strings. When the kickdrum hits the strings duck based on the compressor settings on the left.