Well Maxi chlorians prevent him from dying, effectively:
Amidst the billions upon billions of individual midi-chlorian deaths in Tenebrous' cells were a tiny fraction of midi-chlorians that were not dying.That would not die so long as they inhabited a living host. These especially tenacious midi-chlorians-Tenebrous had privately labeled them with the indeed, Tenebrous had gone to considerable trouble to ensure it would always remain so.
So even if Malgus when Malgus kills him in a duel, he can effectively migrate as a retrovirus into Malgus's system.
In terms of combat applicable feats, they're probably pretty close in terms of barrier/TK feats. No real edge there with Tenebrous mainly being hyped due to being so far up the Banite Sith line, and being faster than Darth Plagueis, and greater telepathic showings/maxichlorians.
He's implied to be an even better duelist that Plagueis, whose handidly defeated Darth Venamis, an ambidextrous master of multiple lightsaber forms that he's able to switch between mid-combat, an apprentice Tenebrous himself trained.
And of course Plagueis passed on that legacy of talent to Darth Sidious.
I think it should be quite obvious that the penultimate generation of the Rule of Two were exceptional masters of lightsaber combat. That makes him more than a match considering he possesses logically superior Force augmentative ability, and can probably calculate Malgus' every move.