how to use the floorplanning options in encounter efficiently

There are multiple options in encounter to floorplan. I'm confused how to use them efficiently. Below are the options I tried and I still couldn't converge on my setup times. My design has got more than 100 macros and lot of guides. I have tired the following options to get best result. Can anyone throw some Idea on how to use the floorplanning options efficiently. First I tried runN2Nopt command after design read in. Then I tired the following options separately.

1. I tired automatic floorplan with multiplan option. Then I select for the best rank. Still some of the paths are getting violated. So I tired createInstance group for the blocks and Still I couldn't get a better result.
2. I tried Plandesign option with default setting. The result is not much of a variant from the above.
3. I created a seed file with the critical soft blocks ( I have selected the blocks deep down the hiererachy where the paths seems critical) as seeds. then I selected the number of seeds as 30 and give selectd the SDP option. My timing got worse.

Can some one guide me how I shouls approch the floorplan problem in an efficient way to  meet the timing with minimum number of buffer added.

How can i choose the seeds efficiently. How is seed selection going to affect my floorplan. Should I really need to specify seed myself or allow tool to take the seed by itself?