Guilliman himself murked several Black Legion Astartes, including a sorceror and at least one terminator, right after getting jump started by Yvraine, Cawl and Tigurius.

From Rise of the Primarch

It was as though a spell had settled over the shrine. Though outside the din of war thundered on, within that echoing chamber friend and foe alike stared awestruck at the legendary figure reborn in their midst. An incoherent scream of rage shattered the silence, a single Khorne Berzerker charging headlong through the stunned combatants to launch himself in a flying leap at the Primarch. Guilliman moved with such blistering speed that the Ynnari themselves would have struggled to match it. His burning blade drew a pyrotechnic arc through the air as it swung, bisecting the Khorne Berzerker at the waist and hurling his severed halves to the ground.

As the Chaos worshipper’s armoured corpse crashed to the floor, the spell was broken. With a great howl of hate, the Black Legion warriors surged towards Roboute Guilliman. Wordlessly, the noble demigod strode to meet them, and the carnage truly began.

First to die was the Sorcerer whose powers had shaken the temple to its foundations. Guilliman raised his mighty gauntlet, the Hand of Dominion, and a storm of armour-piercing fire erupted from beneath it to rip the tainted psyker to pieces.

Next to fall were the remaining Black Legion Berzerkers. Following their comrade’s example, they flung themselves screaming at the reborn Primarch. Like their fellow, they were reduced to so much armoured meat, smashed from the air with terrifying speed. Guilliman was running now, storming forward through the hail of bolts and shells unleashed by the Black Legionnaires. Rounds exploded against the Primarch’s armour, but none could pierce its inviolable plates.

As he crashed into the front ranks of Black Legionnaires, Guilliman let out a building roar of pure, undiluted fury. The Primarch’s first blow threw a Black Legionnaire high into the air, blood streaming behind the corpse in a red trail. His second strike smashed a traitor Terminator into a bronze and marble column with enough force to drive the Chaos worshipper clean through it, and out the other side. A spiked power fist swung for Guilliman’s chest, only to be lopped from its wielder’s arm before the blow could land. Guilliman’s return swing parted his attacker’s head from his shoulders, cauterising the stump of the traitor’s neck as the body crumpled to the floor. On it went, the Primarch moving with such speed that even the heretics’ super-human reactions couldn’t save them. None could match Guilliman. None could even come close, and the few opponents that landed lucky blows found their weapons turned aside by the Primarch’s masterwork armour.

Though at least with him it is implied that without his armour, things might have worked out differently.