He regularly loses in single combat to his brothers.

Lorgar vs Guilliman was actually pretty even as far as Primarch duels go, Guilliman likely could have won if Angron didnt appear.

Angron vs Guilliman was a pretty heavy defeat though it was right after Lorgar too. He also made a woopsy and helped Angron along to Demon Primarch status by stepping on one of the skulls of Angrons slave friends.

Demon Fulgrim vs Guilliman was the loss that left him in stasis.

Demon Magnus vs Guilliman was him getting absolutely slapped even when reinforcements in the way of Custodes, Sisters of Silence, and Imperial Fists are helping. It took someone killing the connection of the Luna Webway gate to stop him.

Demon Mortarion vs Guilliman was in Mortys favor but Guilliman has hax in Godblight.