Always think about where the user would be expecting to find each comment.

The route I would use would be to use Angular in order to display your list of subreddits alphabetically. Selecting the checkbox of the subreddit selects all comments within that subreddit for deletion. Selecting anything other than the checkbox in that subreddit <div> will simply use ngIf to display a <div> below it containing an ngFor <ul> showing each comment within this subreddit chronologically for selection. You can always break these into smaller groups such as years/months/days.

In the case of /r/gonewild I would also allow for deletion of only comments with pictures/links.