not done it myself. with the db-d plug it's easy.

that's what i googled up for the waves c1:

1)add a stereo group track.

2) add the C1 comp-sc as an insert device on the group channel.

3) Now we start the routing. In the inspector, Set the output of the synth's AUDIO channel to the input of the group and set the pan to hard right.

4) For the drum's audio channel put in a pre-fader send to the group (in the channel settings window). Make sure that the send is "on" and that the send level is raised to at least 0db. Now pan the send to hard left (the pan parameter is only visible on the send when in routing mode, found in the little black box at the top right of the channel settings window) We want this to be pre-fader so we can turn down this channel's fader and not change the send's level to the group (compressor).

5) play back, if everything is set up correctly you should hear the synth on the right, the drums on both left and right (although slightly higher volume on the left ( now that it is going through 2 channels). If the group's fader is turned down all the way then the synth should not be heard and the kick (if mono) should be equal on both sides.

6) Now to set up the compressor. For "EQ mode" select "sidechain". For "key mode" select "L --> R" , this sets the kick as the sidechain key and the synth's signal as what is to be controlled by the key.

By the way it's the same procedure for using SX vocoder with an external signal