Hi;
You should get familare with the Apps. Best results i always got in a team knowing Oracle AND the Application.

On the other side look into V$session/V$sesstat to identify the sessions using the largest amount of mem, and then ask on the AppSite what the App is doing.

Have a look on the SORT_AREA_* -Params too they can waiste large memory if they are set to high.

You can control the number of Disk/Memory-Sorts in the v$sysstat-View

Orca