What the achievement asks
Cook Legendary soup at the Community Feast in Iskaara, then defeat Bisquius. Ten points, and it rewards the Big Kinook's Spare Ladle appearance.
He only appears if the feast goes well
The Community Feast unlocks at Renown 3 with the Iskaara Tuskarr and runs for fifteen minutes. The soup has a quality that rises as people complete tasks, and Bisquius only crawls out of the pot at Legendary.
That quality is a group measure, not a personal one. Nothing you do alone will raise it, and there is no way to summon him with a small party. This is the achievement in the branch that depends entirely on how many people are standing next to you.
Which is why it is easy on the right day and impossible on the wrong one.
Pick the day
Reset day, when the feast is the weekly. When Aiding the Accord: A Feast For All is that week's quest, enough people turn up that the soup reaches Legendary in half the time limit. Players who spent a full week failing have gone back on the right reset and had it in one attempt.
A weekend evening. More players, more tasks, faster soup.
With a group you brought yourself. Custom groups form in the group finder for exactly this, and communities exist for the whole of A World Awoken. Be aware that a large group is not automatically the answer: sharding can scatter a raid across instances of the zone, so joining someone who is already at a busy feast works better than assembling twenty people and hoping.
While you wait for the soup
Every task you complete pushes the soup towards Legendary and counts towards Soupervisor, which wants a hundred of them. Press the Yes Chef! extra action button whenever it appears, and throw in every chunk of meat the exploding beasts drop.
Bisquius is also one of the rares in Adventurer of The Azure Span, so the same kill closes a criterion in another branch. If that one is still open, this is two achievements from one fight.
Where it fits
Part of Across the Isles, which rewards the Stormtouched Bruffalon and is itself a piece of A World Awoken.