Use this query on a local DB first and try out:
DELETE Auditing.AuditTransaction FROM
Auditing.AuditTransaction AT
JOIN
Auditing.AuditTransactionItem ATI
ON
AT.SystemId = ATI.AuditTransactionSystemId
WHERE
AT.TransactionDateTimeUtc < DATEADD(MONTH, -3 , GETDATE())
AND
ATI.EntityType <> 'Litium.Security.AuthenticationService, Litium.Abstractions'
“-3” = Month back in time you want to delete
It will not delete Authentication rows.
Remember to restart the Application pool after.