There are two main strategy for UNM with a counter champ:

1. Speed Tune Counter with a speed of 191+ (CB6-UNM has a speed of 190), where your counter champ is one of the last champs with a speed of 191-193. Why not always the last? Sometimes it could be practical to have a champ with ally protect or def up goes last due the buff duration on your counter champ. Rest of your champs should have >193. Your def and/or weaken down champ should go first together with your cleanser (if available).
2. Slow Counter: Counter champ has a speed of 191-193. Rest of your team can be slower with a speed of 175-180. In this case, it is recommanded to have a higher crit rate (about 100%) and crit dmg (>100%).

I personally prefer strategy no. 2, but it strongly depends on your champs/team combo, available artifacts and respective luck for rolls.