He defeated the Jedi Battlemaster as an apprentice. 30 years later he practically defeated Satele Shan in a duel, but was eventually defeated with her Force blasting him and dropping a hill of rock on his head. Despite his injuries, he continued to fight. He went to a nearby city and was attacked by two Jedi. One (Zabrak) Jedi dropped two large buildings on his head with the Force, but he held up the rubble and blew it away. Jumping out he throttled one Jedi and one-shot the zabrak Jedi with lightning, overpowering his lightsaber defense and burning holes in his chest with lightning.
Later at the Sack of Coruscant he hacked through numerous Jedi at once, killed them with Force pushes and slew 3 Jedi with a single blast of lightning. He then killed Ven Zallow (a great Jedi) in a duel. Later he engaged Zallows apprentice, Aryn Leneer. Leneer was fast enough to appear in multiple places at once and slow her perception of time to a crawl, strong enough to throw Malgus 20 meters with one arm and powerful enough to toss a 6 car cargo tram with a gesture and block Malgus' lightning in mid-air (recall, a Jedi who was powerful enough to rip down 2 buildings couldn't block Malgus' lightning). Malgus had something of an upper hand in TK during the fight. During their second fight Malgus reached an epiphany and became one with the Darkside, drastically increasing his powers. Where Aryn could previously block his lightning in mid-air, after that Malgus almost casually subdued her with a single blast. Malgus then completely dominated a Sith called Adraas, at one point kicking him hard enough to send him across a room and crack a pillar in half and dominate him with TK.
He's also destroyed a wave of fighters with a Force Scream, by overloading their fuel slugs.