Before you jump into “The Pain” 
I suggest you review my Onboarding post for newbies and trial users.
Onboarding steps for Xert Trial users and newbies - General - Xert Community Forum (xertonline.com)
Also, consider one of the Fitness Test options in the Workouts Library as an alternative to the Ronnestad micro-interval approach. Search for “fitness test” in quotes.
I suggest riding any Breakthrough test workout entirely in Slope mode, exceeding targets if able, and be prepared to spin up (stand if you want) and go for broke on any longer interval designed to be ridden to failure. Right before you are about to fail, try a max sprint effort for at least 5-7 secs.
The reason for Slope mode is to ensure you can fully express your fitness signature. While many hard workouts can generate BTs, if your signature is understated the workout will be too easy in AUTO mode (ERG control). For example, one way to know The Pain That’s I’m Used To wasn’t tough enough is if you get through all three sets intact and feel you could have done more. Instead, you should feel spent during one or more sets and may not complete all three.
If you do decide on The Pain how you ride the first set will tell you what’s required for the next two. 
Another reason to do the entire workout in Slope mode is smart trainer responsiveness during micro-interval transitions. This isn’t a factor when you control the resistance with gears/cadence.