In Ratchet & Clank: Rift Apart, Rivet and Clank are called back to Sargassum Outpost L51 for the optional objective "Help Trudi". First, Mort will ask you to collect 15 zurpstones in exchange for the Feeding Friendsy trophy, then he will ask you to collect 30 zurpstones for the Ranger Boots, and finally 60 for a Spybot.
After collecting 30 zurpstones, Mort will grant access to Trudi, the friendly dragon who lets Rivert ride on his back. While this allows you to collect zurpstones faster, high zurpstones may seem tempting to collect as you can fly through the air. This is precisely where the confusion lies.
In short, avoid these inaccessible zurpstones until you have collected 45 in total.
While you can technically reach the zurpstones high up with Trudi, you won't be able to collect them. That is, until you collect 45 total zurpstones from the available land zurpstones. Mort will inform you that Trudi has a fireball attack using R2, but he won't say anything until you collect 45 zurpstones on foot.
Death grants Trudi access to 30 zurpstones, but the game doesn't do a great job of telling you to collect the ground zurpstones first. Before the fireball ability is unlocked, Trudi is more useful for traversing the map faster, as opposed to traveling on foot. There are certain places on the map where she can perch (press the O button), allowing you to drop down and explore the surroundings.
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Many of these zurpstones are grouped together in the same area, which will help you collect a bunch of zurpstones in a short time. But these clustered locations often result in defeating a Grunthor, so be prepared to take them out using every weapon in your arsenal.
The game teaches you Trudi's commands pretty quickly, so here's a breakdown of her abilities:
- L1 or L2 (Hold): Brake
- R1: Increase speed
- Lens: right analog stick
- R2: Fireball (unlock at 45 zurpstones)
We were one of the players tempted by high zurpstones before Mort informed Rivet of Trudi's fireball attack, and wondered why we couldn't get them back. So just be aware that you can't walk up zurpstones high up, nor pick them up until Trudi's fireball ability is unlocked at 45 zurpstones.
For more Ratchet & Clank: Rift Apart guides, check out Ratchet & Clank: Rift Apart - Shatterbomb or The Enforcer? on Pro Game Guides.