Possible to implement simple IIR filter with greater than 6 db/octave slope?
You need a filter with a higher filter order, which means that your filter memory must be longer. The current output sample $y[n]$ is computed as

$$y[n]=a_1y[n-1]+a_2y[n-2]+\ldots + a_Ny[n-N]+b_0x[n]+b_1x[n-1]+\ldots + b_Nx[n-N]$$

where $x[n]$ is the current input sample, $N$ is the filter order, and $a_i$ and $b_i$ are the filter coefficients. You can design the filter (i.e., compute the coefficients) using some tool like Matlab or Octave.