Jira (8.0 - 8.14)
import com.atlassian.jira.issue.priority.Priority
import com.onresolve.jira.groovy.user.FieldBehaviours
import groovy.transform.BaseScript
@BaseScript FieldBehaviours fieldBehaviours
def priorityField = getFieldById(fieldChanged)
def customField = getFieldByName("Number Field")
def priorityValue = priorityField.value as Priority
priorityValue.name == "Highest" ?
customField.setRequired(true).setHidden(false) :
customField.setRequired(false).setHidden(true)