If we need a change an xml message with specific condition of an existing error, in an example of error response seeing below, we can use Groovy with XmlSlurper.

Sample Message:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
    <soap:Body>
        <soap:Fault>
            <faultcode>
                soap:Server
            </faultcode>
            <faultstring>
                javax.xml.ws.WebServiceException: com.exception.BaseUserException: Some Error.
            </faultstring>
        </soap:Fault>
    </soap:Body>
</soap:Envelope>
XML

Groovy Expression:

import groovy.xml.XmlSlurper
import groovy.xml.XmlUtil

var rootNode = new XmlSlurper(false, true).parseText(responseBodyTextFromTargetAPI).declareNamespace('soap':'http://schemas.xmlsoap.org/soap/envelope/')

def element=rootNode.'soap:Body'.'soap:Fault'.'faultstring'

if(element != ''){
rootNode.'soap:Body'.'soap:Fault'.replaceNode { }
rootNode.'soap:Body'.appendNode {
  faultstring(element.text())
 }
 
 responseBodyTextFromTargetAPI = XmlUtil.serialize(rootNode)
}
GROOVY

Result:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
    <soap:Body>
        <faultstring>
            javax.xml.ws.WebServiceException: com.exception.BaseUserException: Some Error.
        </faultstring>
    </soap:Body>
</soap:Envelope>
XML