The danger of the tarrasque is in its lore. Unless testing your skills to make dynamic duos or tanks that can handle a Tarrasque on their own at level 20, it lacks a bit of oomph if you use it purely as a gladiatorial fight for a party of four. I would personally suggest testing your party out with a gauntlet across the whose who of horror. The Lords of the Nine Hells led by Asmodeus. Maybe even the Demon Princes led by Demogorgon, the Prince of all Demons. One by one, preferably, taking them on as an actual army of Hell or the Abyss sounds like a nightmare. Easier to spur infighting anyway.