Go to the herbivore island (right bottom corner of the screen). There's a big entrance inside of a hill, with a lot of free space. You have to block the entrance so that your brontosaurus can't escape. You can rather wait until it spawns inside of your trap or lure it. When it's inside and can't get out,take the high ground and tranq it. It'll be way easier since it can't hit you with it's tail, after that you can simply use berries to tame it. I'm writing this while taming a bronto, it works.
I recommend taming this as soon as you get the saddle! They’re a super easy tame, use a crossbow with about 25-80 depending on the level, hop on a pteranodon after shooting it once, land, hop off, shoot it more, rinse and repeat till KO. If you have boosted taming time, just use mejos and it’ll work fine. Up if this helped!
These are the easiest tame ever, just get far away, shoot it with a crossbow repeatedly, when it gets close run and shoot, then boom u knocked out a brontosaurus (I literally have 5 of them that’s how easy they are to tame) please