So far, there are two people who face Jiren. One is the strongest fighter Universe 7 (Son Goku) the other strongest fighter Universe 6 (Hit, who once killed Goku in one shot). Both of them even failed to make Jiren's costume damaged.
If Hit and Goku alone can not face Jiren, who can?
The author thinks there are still six fighter who, in the right circumstances, have an expectation against Jiren. These are the special fighters of the Tournament of Power!
Frieza is one of the most prominent fighters of the Tournament of Power so far. The amount of elimination is quite a lot, and it's still pretty fresh. The reason he is not one hundred percent is because he contributed his stamina to Spirit Bomb then, with a little nge troll, healed Goku.
In addition, Frieza is not hurt because he is always targeting weaker enemies. So he was never hit hard as Goku, Gohan, and even Vegeta almost absorbed Mafuba.
Frieza tells Goku that he is not interested in fighting Jiren in episode 111. But this is Frieza. This one creature always has a cunning plan, as he pointed out to Frost of Universe 6.
According to the author, Frieza is just looking for an opponent who can weaken Jiren. (Like Goku), so he can face Jiren when Jiren is weak. It could be that he is also secretly trying to learn the Ultra Instinct, so he can take revenge to Beerus.
When sparring with Goku, Gohan completely relies on a Mystic / Ultimate form. Gohan said that he was looking for his own way to be strong, so he tried not to use Super Saiyan.
So far, Gohan proves his determination by purely relying on the Ultimate form in the face of any enemy in the Tournament of Power. Given this form is as powerful as Super Saiyan Blue, it is not surprising that he prefers to use it rather than his Super Saiyan form which stuck in Super Saiyan 2.
If Gohan is able to further develop his Ultimate form, it could be that he will be strong enough to duel with Jiren.
Two other fighters of Tournament of Power who have hope against Jiren can you read on the second page.