Impossible, also referred to as High Level or HL, is content that becomes available after reaching rank 101. Players are initially capped to rank 100, but can raise the cap by completing the quest Twinkling Stars (20 AP) on Zinkenstill.
Twinkling Stars pits you solo against weaker versions of each Omega primal beast.
They have mostly the same mechanics as their Omega versions, so refer to the Omega strategies if you are unsure of how to defeat them.
Most of the same content is available at HL, although there is some new content (Rose Queen). Mainly, HL versions of old content becomes unlocked, with new HL items dropping that are needed for many final uncaps.
HL content of note: