Try zoning to different zones. I had this problem and after a bit of fooling around and zoning back and forth I was able to switch my guilds.
It might not work for you but it’s something to try at least until a permanent fix is implemented.
Personally I love the giant bosses. As a Thief I just use Death Blossom or roll and he can’t touch me. I wouldn’t change a thing about the fight mechanics. If you can’t melee him effectively than use range.
I do agree thought that the giant in Diessa Plateau needs a chest or something as it is a lot of effort for 80ish copper.
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