Ducking in Sonar is pretty easy:
1. Insert a bus, call it Ducker.
2. Add a Sonitus compressor to the Ducker FX. Start with the drum destroyer preset with an 1/8th note release time (234 ms == 128bpm).
3. Patch the channels (bass, strings, etc) you want to duck to the Ducker bus.
4. Create a send on the audio channel (kick) you want to use to drive the ducker and point it at the Sonitus side chain input. I usually bounce a couple measures of kick and paste it where I want things to duck.