setup timing fixing after postroute

Hello all,

I'm beginner in PD. i have database which is already post routed. and i have to fix setup violation for one particular clock. actually there are 4 clocks which showing negative slack. i have some questions please suggest solution on this.

when verified for timing am getting 3000 + setup violations.
1) In some paths, I cant upsize the cells as the cells used are of maximum drive stregnth, in that case what should i have to do?

2) i find that some paths are not optimized after post route, so how to optimize those particular paths?

3) If there are unnecessary buffers in data path that are not affecting other paths and if they are not affecting hold violations in that path, can i remove them? And
How to know whether the buffers inserted are unnecessary or not and do not affect other paths?

4) how to search common paths?

5) Upsize/Down sizing the cells: by trying to upsize common cells in common path that max path, is it reduces setup violations? or in which cases we have to downsize cells?

i tried to upsize cells, Initially it helps to reduce slack but after a point, upsizing actually degrades the delay.

Thank you.

1- is the design is timing clean before the postroute?
a- if yes, it is also DRV clean, is there huge routing hot spot, which could have been not properly handle before?
b- if no, fix the setup before routing.