You need to get hold of a decent text on filter design - I use the
"Electronic Filter Design Handbook" by Arthur B. Williams and Fred J.
Taylor - ISBN 0-07-070434-1. Since then there has been a third edition
(ISBN 0-07-070430-9) which is supposed to be even better, but it seems
to be out of print -
www.amazon.com offers three second hand copies for
about $130. You should be able to find it - or something very like it -
in a university library.
The usual choice for what you seem to want to be doing is a
"linear-phase" or "Bessel filter", though a "synchronously tuned"
filter (consisting of identical multiple poles) actually gives fastest
settling for a given number of poles, with the step getting to better
than 99% after five time constants. Adding more poles makes the step
steeper at the 50% point, and pushes the 50% point back from around one
time constant at one pole to closer to three at ten poles.
A simple LCR filter is two poles ...