First of all, you have provided very little information about the terrain. One option is that you should find a spot where you have a clear line of sight to all the other access points and put a 5 ghz omni in there. Then, at the local base stations, you install one 5ghz directional panel and a 2.4ghz omni for the clients. Another option is to install four 2.4ghz sectors in one spot (assuming you need to cover a 360 degree area around it). But without some additional info about obstacles it's impossible to say what would work better.