Irithyll of the Boreal Valley

Before reaching the Central Irithyll bonfire, you would be harassed by a Sulyvahn's Beast as you cross the bridge to Irithyll. Sit at the Irithyll of the Boreal Valley bonfire and attune Chaos Bed Vestiges. Equip these rings to boost your fire damage output: Witch's Ring, Great Swamp Ring, Lloyd's Sword Ring. For the fourth ring slot, you can choose to equip the Fire Clutch Ring or the Chloranthy Ring+3. Before entering the bridge, cast Sacred Oath or use the weapon art of the Sunlight Straight Sword, and cast Deep Protection. Now run to the designated location as shown in the video below. Once you're at the designated location, the Beast would pounce at you. Before it lands on the ground near your location, lock on to the Beast and cast a Chaos Bed Vestiges. This would stagger the Beast for an easy riposte. Cast another 2 or 3 Chaos Bed Vestiges to finish it off. Now light the Central Irithyll bonfire and sit at the bonfire if you need to.

Sulyvahn's Beast cheese at high level, Dark Souls III

From the Central Irithyll bonfire, run up the staircase. Cast Hidden Body and use the Ring of Favor glitch to sprint past the first group of 3 Irithyll Knights. Continue dashing your way to the Church of Yorshka bonfire, casting Hidden Body on the way as necessary. Once you have reached the bonfire, light it and sit at the bonfire if necessary. Now use the Ring of Favor glitch to sprint to the Distant Manor bonfire, light the bonfire, and run to the arena of Pontiff Sulyvahn. After entering the building where you would meet the first group of 3 Silver Knights, cast Hidden Body and sprint past the Silver Knight standing on the ground floor. Exit the building, head up a flight of stairs, cast Hidden Body, and sprint past the Starved Hounds. Also use Alluring Skulls to distract the Starved Hounds.