Relative detection range vs RCS
Known scenario = X

Problem scenario = Y

Detection Range Y = Detection Range X / (RCS X1/4 / RCS Y1/4)

Assume Su 30 MKI and J20 with the same radar detection rang for non stealthy jet,  170 km,  assume J20 has RCS 0.001 (10  times bigger than F 22) , RCS for Su-30 MKI is 20, 

then, J20 can radar can detect Su30 MKI 170 km, but Su 30 MKI can detect J20 at 170/12.28=13 km.  in air battle, J20 fire PL 15 missile (range 175 km) at 170 --------------------------50 km, and shoot Su 30 MKI down, Su-30 MKI will not be able to detect J20 . 

Rafale is not much better, since its RCS is 10 with 6 missiles, so then, J20 can radar can detect Rafale at170 km, but Rafale  can detect J20 at 170/10=17 km.  in air battle, J20 fire PL 15 missile (range 175 km) at 170 --------------------------50 km, and shoot Rafaledown, Rafale will not be able to detect J20 until 17 km,

with this, you can even make J20 RCS even bigger, as long as it is in the missile range and outside detection of Su30 MKI or Rafale, J20 will be able to shoot them down, not being detected by either of them.