Looks like ClearTable is not the correct method. As per the code, its doing

ClassMethod %ClearTable(pTable As %String)
{
Kill ^Ens.LookupTable(pTable)
}

so..i did this but not too comfortable touching the globals directly. Hoping there is a cleaner way... (the below works)..

Table initialization

s tmpTable = ##class(Ens.Util.LookupTable).%New()
s ^Ens.LookupTable("myNiCTmpTable00992314","DUMMYVAL")=0 //create some randomly named table

<....use this tmp table for lookup and adding new rows.>

delete the tmpTable once done with usage

d tmpTable.%ClearTable("myNiCTmpTable00992314")

Edit: Adding a 2 digit random number at the end of the temp table name to ensure no conflict if multiple threads are running at the same time.

Thank you Eduard. Below is the error i am getting

ERROR <Ens>ErrException: <METHOD DOES NOT EXIST>zSetPatientPhotoOperation+2^UH.Photo.Operation.IPatientAccessMobileServices2016.1 *SetHttpHeader,EnsLib.SOAP.OutboundAdapter -- logged as '-' number - @' Set tSC = ..Adapter.SetHttpHeader("something", "someValue")'

Thank you Eduard and Robert. 

Do i have to encode this to base64 prior to the WS call?