The rumors that Plagueis was a weak duelist are unfounded; he's a master duelist per Tenebrous and his opponent, Venamis, is proficient in many forms of combat and skilled enough to switch dueling hands mid-battle.
Venamis attacks Plagueis on orders of the late Tenebrous. Venamis is essentially portrayed to be equal to Plagueis, except that his enthusiasm for the dueling arts is more prominent. They duel and he initially gains the upper hand and nearly decapitates Plagueis, who in turn expands his awareness to an extent that he essentially fights on autopilot and goes on the defensive, forcing Venamis to expend his energy trying to breach Plagueis's defense. {At one point, seeking the high ground, Venamis leaps onto a tree branch to attack from above-- Plagueis destroys the branch, and Venamis uses the Force to remain in mid-air.} Finally, Plagueis disarms the exhausted Venamis. Venamis concedes the fight and submits himself to Plagueis, who forces him to devour a plant. Venamis says it will poison him and Plagueis says, "Yes, but I'll keep you alive."
I'm currently in the midst of a serious read through and I'm taking relevant notes. Martially, Plagueis is very skilled. He's capable of lifting enormous stone slabs that interred his and Tenebrous's starship; he's capable of absorbing blaster-bolts bare handed and regurgitating them as Force lightning; he's a skilled martial artist powerful enough to hurl one of his opponent's across the ship's hold with a single kick; his pain threshold is such that when two of his enemies attack him with teeth and claws, there is "no imperceptible effect"; and he kills one of his enemies instantaneously by snapping his fingers.
The Gran Protectorate put a contract out on Plagueis. Taken by surprise, he's severely wounded by a sneak attack from the assassins, though he manages to fight back against four dozen or more assailants (48), who are said to be skilled enough to have "wounded and killed Jedi." His counterattack is impressive, despite two of his three hearts failing from the wounds: He clapped his hands and "turned every loose item into a projectile" in the vicinity; loosed a Force scream that shattered the eardrums of those within ten meters; and conjured a Force wave that atomized six of them instantly and then uses the same wave to kill six more.
A Force Wave that kills 12 Jedi-level opponents, atomising half of them, sounds out of Malgus' league imo.
Hardly obscure. And I'd prefer to read the actual passage before jumping to claims, but taking on 50 jedi-level combatants and surviving is an unprecidented feat. And atomising 6 beings with a Force Wave is telekinesis beyond Malgus. Its enough to put Plagueis on Bane's level of offensive Force prowess, which is a level well beyond Malgus. IMO, Plageuis is a combatant who should be able to manhandle Baldy McNogirlfriend.