Well, Malak was one of the better students the Order had seen and also was the strongest Sith besides Revan out of thousands. He had years of combat experience, especially fighting against other Force wielders instead of just droids. It is difficult to make a really solid comparison because of the lack of any mutual opponents, but I think it's fair to say he's probably stronger than either of them individually. Together though, they should take him down. And by should, I mean almost certainly will.
Malak nearly killed Kavar, the famed Jedi Guardian that even the Mandalorians had heard of.
He was Revan's second in a Sith Empire where "the strongest ruled".
His duel with Revan is notably epic and not "Revan steamrolled him and/or used mines lol", according to Drew K.
Malak demonstrates the ability to Force freeze multiple Force users, something which isn't demonstrated by any of his foes in this fight. He could conceivably freeze them all and cut them in half.
Malak's wartime experience molded him into one of the foremost Jedi combatants of his time according to in-game sources (KotOR 1 and 2).
Nowhere is he demonstrated as being weaker than Fisto, whose achievements include being incapacitated by Ventress in seconds and dying to Sidious in seconds and Plo Koon who, while godly in the ability to freeze creeks, is noted as being an exceptional pilot.
It's clear that Malak is not a random mook, and that in an era where lightsaber combat is extremely common (Compared to the PT), he ranks above almost everyone (Revan really being the only exception). Whether it's through superior saber technique or raw Force talent (Or both), a bonafide Sith Lord is likely too much for Fisto and Koon.