The Celestial Tournament is a special instance and by most seen as the pinnacle of pet battle challenges.
You can do it once a week and inside, you will face three pet battle masters, followed by the four Celestials to fight against. The three masters change every week and there's a total of three sets of them rotating through.
Doesn't sound that hard, but what makes it tricky is that once you're in, you cannot heal your pets anymore! That makes planning ahead very important.
Xu-Fu can scan this section for the most suitable strategies according to your preferences and show the required pets.
It is recommended to check the tables week by week since you won't have to face all 9 tamers in one run.
The tool will use every pet only once due to the restriction of no healing.
Active: US: Week 1 with Taran Zhu, Wrathion and Chen Stormstout EU: Week 2 with Shademaster Kiryn, Wise Mari and Blingtron 4000