You'll need to make 2 measures for this one.
The first will be easy. Make a measure of Visit Count and give it a Upper Severe Threshold of 20.1 (which will trigger for greater than 20). Add that measure to your incident and make sure the aggregation is count.
IGNORE, SEE EDIT:
The second measure will be of Visits - User Experience Reason, just search that when you go to create a measure and it'll come right up. Set match to "contains" and value to "frustrated". To make this measure trigger on a percentage rather than count, click on "details" near the top. There you can change the units from number to percent. After that, click on the configuration tab again and change the upper threshold to 25. See images at the bottom in case you get confused.
Add this measure you've created to your incident as well, make sure the aggregation for this one is average. Now that you have two conditions, you want your logic to be AND.
That should be all you need to do, make the Severity and time frame whatever you'd like. I've included a screenshot of what the incident should look like when you're done.
Seeing as Visit - User Experience Reason will not work, the only other solution I could come up with would be to use User Experience Index (Visit) instead.
I'm not entirely sure what is all used to calculate this number (if anything), but I noticed at my customer site that the UEI for frustrated visits was always = 0, tolerating was always = 0.5, and satisfied was always = 1.
If this is the case, you can create a measure for User Experience Index (visit) and set the Lower Severe threshold to 75. This rule, along with the 20 or more visit count will trigger under similar conditions as the one you described, as if at least 25% of visits have a UEI of 0 the UEI should be 0.75 or less. However, this can be incident would be triggered by a significant amount of tolerating visits as well, 50% tolerating visits would trigger it.
I've included a screenshots (in order to get the incident to trigger more quickly I increased the threshold for UEI that triggers the incident.
That's the best I can do, I hope it helps.
Thanks @Trevor M.for the quick response! That was extremely helpful.
When I created the incident though I see a warning by it and the hint - "This Incident Rule has no effect since at least one of its Measures is configured not to create a timeseries."
I know it's the 25% frustrated users measure, it must be because it's an Evaluation/Filtering/Splitting measure. Do you get this warning or is there some configuration to store these values?
Thank you again for your help with this!