Does a Legendary Aniipod guarantee a capture?

The guarantee applies only after you win the fight, and only with a pod you crafted. You defeat the Legendary Aniimo first. Then, during the soothe interaction, you throw the Legendary Aniipod you made beforehand and the bond forms every time. There is no catch rate to roll against and nothing to retry.

There is also no second attempt to farm. The narrative boss encounter against Irisalis can only be completed once, and crafting the pod is what starts it: the pod detects Irisalis's trails and opens the rift space where you fight it. The fight is not difficult. It is easier than the weekly investigation boss.

Getting the Irisalis pod and the second legendary Anipod

Irisalis comes from the Windchasers Departure event in the Legendary Journey section of the event menu, and you can only get it during Season 1. Collecting Irisalis Petals reaches the milestones that unlock its encounter.

Once you have crafted the Irisalis Anipod, you can spend more petals on a second legendary Anipod, Wind Chaser's Departure. It also never fails. An Aniimo caught with it always appears in sparkling form with perfect innate potential.

For more petals, run Memory: The Faded Rainbow from the Weekly Rotation tab. It is a legendary refight that costs energy and pays 3 petals per clear. You can clear it twice per reset, for up to 6 petals, and anything you leave unclaimed is lost when the reset passes.

Not settled: The reset period for Memory: The Faded Rainbow is not settled. It is listed both as weekly, on Monday at 4:00 a.m., and as every 3 days, and that changes how quickly you can collect petals for the second pod.