i found solution. (i think)
<S:Envelope xmlns:S="http://schemas.xmlsoap.org/soap/envelope/">
<S:Body>
<ns2:uiCallMWResponse xmlns:ns2="http://ui.ws.mmw.iskratel.si/">
<return>
<bIsExecuted>true</bIsExecuted>
</return>
<xParams>${xparam_value}</xParams>
</ns2:uiCallMWResponse>
</S:Body>
</S:Envelope>
def groovyUtils = new com.eviware.soapui.support.GroovyUtils( context )
def holder = groovyUtils.getXmlHolder( mockRequest.requestContent )
holder.namespaces["ui"] = "http://ui.ws.mmw.iskratel.si/"
String xparam = holder.getNodeValue("//ui:uiCallMW[1]/xParams[1]")
i'm not sure if i understand this situation 100% but it's working and that's important right now.