Here is a very simple step by step guide to getting a sidechain feel inside Sylenth 1 without a compressor.
1. Turn off OSC 1 [A1] by putting zero in the voices section
2. Go to LFO > Choose 3rd lfo choice which is (ramp)
3. Turn the LFO rate to 1/4
4. Bring the LFO gain all the way up
5. Choose Mix A or or AB as the LFO destination
This will give you a pumping effect!