For your convenience, there are a few ways to access the feature.
A) Press the image of the Trinity on your Home page, then choose the Dark Lord you'd like to spar with.
B) Press Fight, then press Trinity. From here, choose the Dark Lord you'd like to spar with.
C) If you've purchased the Quick Links addon, you can long-press in the Player Info bar (black bar at the top of every screen) and choose to go directly to any Dark Lord's profile.