Assign the issue to the user who last commented on the issue.
Select Current issue.
The current issue will be updated.
Select Assignee.
Set the Value* to Set field value manually (parser expression) and insert the following expression:
%{issue.lastComment.author}
The user who last commented on the issue will be set as the new assignee.
Choose which user* will be able to execute the post function. By default, it is set to the Current user.