In order to trace sessions created by a specific user, you can use following login trigger:
CREATE OR REPLACE TRIGGER tgardba.TRG_SESSION_TRACE
AFTER LOGON
ON DATABASE
DECLARE
sqlstr VARCHAR2(200) := 'ALTER SESSION SET EVENTS ''10046 TRACE NAME CONTEXT FOREVER, LEVEL 12''';
BEGIN
IF (USER = 'USERNAME') THEN
execute immediate sqlstr;
END IF;
END TRG_SESSION_TRAC;
/
