Is it sufficient to supplicate for someone you have slandered as a form of repentance, instead of praising them in the gatherings where you slandered them, due to the difficulty of doing so?

It is not a condition for the validity of repentance from backbiting that the backbiter mention the person he backbit and praise their good qualities in gatherings, nor is there any Shar'i (Islamic legal) evidence for that. This opinion was narrated from some of the Tabi'in (successors of the companions), while others hold that seeking forgiveness is sufficient, citing the hadith: "The expiation for backbiting someone is to seek forgiveness for them." This is merely an opinion of some scholars and not an explicit matter attributed to the Prophet, peace and blessings be upon him. When scholars mention this, they intend it as a recommendation.