When you have dozens of ad sets running, it's essential to use automated rules. That way you won't need to go through your ad sets to find those that deliver poorly and stop each one of them manually.

When your Cost Per Acquired customer is 2x or 3x times greater than usual in the last couple of days – it's time to pause that ad set.

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When you know the limits for your Cost Per Link Click or Click-through Rate it's easy to come up with simple pause rules: 

Please note: additional conditions with Spend and Impressions help you get enough statistical data to evaluate CTR and CPC more precisely. If there is no data all metrics equal zero by default. 

Return On Ad Spend (ROAS) = Purchase Revenue / Spend
This is a powerful metric used by professional marketers as an alternative to ROI, as it shows direct relation between your ad spend and revenue.

You can pause your ad sets at certain times during the day when they do not perform well.  Use Time Greater and Time Less option.

The example rule above would pause your campaign at the end of the day in the period from 8 to 9 PM. You can start it back again the next day at any time.
You can also set the rule to run at a specific time using the custom schedule in the schedule section at the bottom of a rule. 

These are just general examples. Make sure to experiment with different values, metrics and rules to automatically pause under-delivering campaigns.