how to setup cloudwatch alarm for Autoscaling group when its scaling down Mincapacity Instances using cloudformation template.
I mean need alarm when all the Instances were "OutofService" basically this will happen when Instance failed ELB healthcheck.
Why don't you add an alarm based on the CloudTrail metric HealtlyHostCount? If you set a low threshold, you will get warned when there are no healthy instances.
You can see the metrics documentation here