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Easy once you know how to parry, but the worst boss for any sort of casters cause his second phase is so aggressive.
Magic is for scrubs
Strategy 6 (Sword Master Decoy) For anyone who not good with rolling and parry, I beat him by keep Sword Master alive as decoy. Summon Sword Master and try to keep Gundyr focus you, make sure that you and Sword Master are always on opposite side of Gundyr. Sword Master will attack Gundyr, he will turn to face SM, run to Gundyr attack him from behind 2-3 time. Gundyr will turn to you, back away from him. Sometime he will keep attack SM so you need to attack him more or SM will die in only 3 hit.
CCCCHHHHHHAAAAARRRRRGGGGGGEEEEEEE
In NG he was beast (I was using lightning weapon and died few times), but now in NG+... surprisingly easy. Just use Vordt's Great Hammer (two handed ofcourse) and spam him with many attacks at the biginning (before he stand up and shortly after) Probably I depleted his stamina rapdidly. Anyway - Vordt's Great Hammer is great weapon against this boss because he is weak to strike and frostbite. Sooo... I think THIS is best option (not parrying)
what a chad
Keep in mind you, maybe soul level 100, are fighting a soul level 1 gundyr,
I summoned Sword Master and this guy just rolled to attack some trash and ran off the cliff, much like how I defeated him in the beginning lol