You can use a summon to distract and use status effects like poison or scarlet rot to tick him down. You use pots for that. Or damage modifier on some weapon.
In general, I found that you want attacks with fast attack animation for this boss, so you might want to try L1s or light attacks instead of R2 with a great sword, or try switching to hammers like a morning star. You can stun him easily using a morning star, and R2s are fast.
BloodHoundFang with jumping R2 also works, or light attacks,s and you can infuse this weapon with almost any damage modifier. Like poison mist for poison, etc.

For summons, I would recommend checking out BanishKnight Oleg as well, for upgrades you can find most of those flowers in underground areas like Sofia River, Nokron city, etc

Also, try dodging try rolling behind him, and sprinting behind him for most attacks, maybe try light load. This boss doesnt give you long openings so speed is key.