Fight takes place on Mustafar. Nihilus does not have his "eat" ability or essence transfer. In terms of feats, this is what we have for Nihilus:
Curbtomping Darth Sion (Cut content, but it has been rendered canon)
Able to do some mild acrobatics in his duel with Meetra Surik and Visas Marr
Force Stasis, which he used to trap Mandalore, Meetra, and Visas. He also used Force Stun (different power) on Meetra at a different point.
Force Drain (something of a specialty of his)
Capable of lifting a shattered capital ship off of a broken planet (The Ravager). Evidence points to Nihilus holding it together through his own sheer force of will. He was also the cause of the ship's travel through hyperspace, as well as a source of life for his minions on the ship. Notably though, he tends to not use TK in combat as much, but he has few combat showings anyway. He did use it on Darth Traya.
Force Choke-Used it on Visas and Sion
Now for Darth Malgus:
Force Lightning that took down three Jedi who had their lightsabers up
In the epilouge of Deceived, he breaches Lord Adraas' stronghold, kills everyone in the complex, and takes down Lord Adraas with his lightsaber disarming him with no harm to his person, and then strangling him to death with his bare hands.
Killing both Kao Cen Darach and Ven Zallow, the latter of whom was the Temple Swordmaster at the time
Force pushed Ven Zallow with enough force to blast him through rubble
Doing quite well against Satele Shan, albeit as both a Padawan and a Knight, and not as Grandmaster.
Special Thanks to Silver and JediXMan's Nihilus Vs. Exar Kun debate for his feats.
Who wins the duel?