These guys are pretty easy to tame. I normally find them by the swamps and it’s quite dangerous there so get a club and hit it in the direction out of the swamp then try to corner it into some rocks or something then knock it out and feed it tinto berries, tamed pretty quick for me. Get a LOT of berries if you are planning on taming one of these guys.
Here's how to tame a Phiomia, multiple steps, with Argentavis and without:
•Have a Bola ready
•Get a club or slingshot, tranq arrows do just fine as well
•Gather PLENTY of crops and/or berries
•Search in the redwoods or swamps, they mostly hang around in those areas, but BEWARE! It's dangerous! If you're lucky, you might find one at the edge of a beach.
•If you found one, wrap it with a Bola
•Use whatever you have to knock it out with
•Guard it, everything will want to eat it
•Have a saddle ready
•Use whatever KO tool(tranq arrows or club recommended)
•Mount the Argentavis
•Search redwoods or swamps
•Pick it up
•Place it in a pen or a safer, secluded area
•Wrap it if not in a pen
•Use tranq arrows or darts for quick KO
•Guard it with your Argentavis
These aren't very good choices as mounts, but can be very useful for farming! When feeding stimberries to them, it acts as a laxative, so you can get fertilizer more frequently! Just be sure to have plenty of stimberries, and enjoy your Phiomia!
Super East to tame, I just cornered it and clubbed it till it was unconscious. Then I feed it, that was the hardest part. I had to harvest berries while taming, so be preped if you want to tame one of these suckers, or just knockout near lots of bushes.