public with sharing class CaseTriggerHelper {

    public static void updateCaseStage(List<Case> cases) {
        
             List<Case_User__c> cuser = New  List<Case_User__c>();
             Map<ID, Case_User__c> cusermap = new Map<ID, Case_User__c>();

             Set<String> skillSet = new Set<String>();
             List<String> skillSetList = new List<String>();
    
             for(Case c: cases){
        
                   if(c.Status == 'New') {
                   cIds.add(c.Id); //add case ids.
                   }     
                   skillSet.add(c.CaseRelatedTo__c);
             }   
           skillSetList.addAll(skillSet);    
        
         //available users               
         
            List<Case_User__c> availableCaseUsers = [SELECT AvailableUsers__c, Timings__c, SkillSet__c       FROM Case_User__c where Availability__c = TRUE ORDER BY Last_Case_Allocated_Time__c ASC];
         //List<Case_User__c> availableCaseUsers = Database.query(queryString);
            integer i=0;
            integer nomatchingSkillset = availableCaseUsers.size();
            for(Case c: cases){
                for(; i<=availableCaseUsers.size();i++){
                if(availableCaseUsers.get(i).SkillSet__c.contains(c.CaseRelatedTo__c)){
                  c.OwnerId = availableCaseUsers.get(i).AvailableUsers__c;
                  availableCaseUsers.get(i).Last_Case_Allocated_Time__c = DateTime.now();
                  cusermap.put(availableCaseUsers.get(i).id, availableCaseUsers.get(i));
                 } else {
                      nomatchingSkillset--;
                      if(nomatchingSkillset == 0){
                          //TODO assign to case queue
                          
                             User u =[SELECT Email FROM User WHERE Id IN (SELECT UserOrGroupId FROM GroupMember WHERE Group.DeveloperName ='CaseQueue')];
                             String us = u.Email;
                                                  
                              Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
                              String[] toAddresses = new String[] {us};
                              mail.setToAddresses(toAddresses );
                              mail.setSubject('No Users available to handle this case');
                              mail.plainTextBody='handle this case';
                              Messaging.SendEmail(new Messaging.SingleEmailMessage[] {mail});
                                                  
                          break;
                      }
                      continue;
                }
                if(i>=availableCaseUsers.size()){
                    i=0;
                }
                break;
             }      
         }
        
      
 
     if(!cusermap.isEmpty()) {
      update cusermap.values();
     }

        
   }
}
​}

trigger CaseTrigger on Case(before insert, after insert, before update, after update, before delete, after delete, after undelete){
    if(Trigger.isUpdate && Trigger.isAfter){
        CaseTriggerHelper.updateCaseStage(Trigger.new);
    }
}