Vendrick is one of the most frustrating bosses in the entire game due to the fact that while he is indeed slow and his movements predictable, one wrong move and a single hit will send you to your demise. Vendrick is also protected by 5 Souls of Giants which massively increase his defence bonus.The key is to collect these souls. Having none in your inventory will give King Vendrick 32x his normal defenses. Having 1 gives him 16x his defenses. Having 2 gives him 8x def, 3 gives 4x def, 4 gives him 2x and having 5 results in him having normal defense stats.
Preparation on this battle is more important than any other. If you want to stand a chance you must get rid of his defence bonus. The guide above details how to get them.
Do not attempt this without at least 4 of the Giant souls, if you can also defeat the ancient dragon and collect his soul then Vendricks loses ALL of his defence bonus. Defeating the Ancient Dragon is a piece of cake, all you have to do is have a fire resistance of 900 this will null his fire attacks. The spell Flash Sweat adds 300 fire resistance so then just upgrade the smelter demon or or chaos sets.
Customise your attributes to this specific battle. I would recommend visiting the witches at the Things Betwixt bonfire and reallocating your stats. Raise your health and endurance to 50 or as close as you can. The high health means you will survive his strongest attacks and the extra stamina is important to utilise every opportunity to attack or avoid danger.
I really recommend raising your Health and Endurance as high as possible. I further bolstered these with the life ring 3 and Third Dragon ring and his attcks only took around half my health meaning I wasnt panicking.
Having a fast high damage weapon is also important I tried literally everything and found that a maxed out Red Iron Twinblade infused with lightning was amazing and was taking 600-700 health per hit and I could do 2000 damage in a combo. This weapon scales with strength so I raised strength as much as I could while keeping dexterity as low as possible. As I infused with lightning which scales with faith. I also raised faith while keeping intel at a minimum.
There is a very simple trick to defeat Vendrick and that is to You need to stay behind him, hugging his left leg. Get in as close as to the leg as possible. If you keep yourself as close as you can to his left leg then he can never hit you. While your behind him don't attack, just wait, Keep hugging that left leg eventually he will do a three part move with two side slashes and then slamming his weapon into the ground. This is your chance, At this point attack, get at least two to three hits in. Don't expend all your stamina and make sure that when he recovers you keep hugging his left leg. Now just rinse and repeat, remain patient, don't try and get to cocky, hug the leg, wait for his 3 piece move set, then attack.
After each attack, Breathe, Retain your composure, be patient, time your rolling and aim for the left leg..
Remove his defence bonus
Customise your stats
hug the left leg
attack 2/3times when he slams his weapon.