While megatheriums and rezes are good to fight them, I’d say wyverns are the best, I would recommend blood wyverns because they’re easy to get while being silly strong for how early game you can get them, but of course, don’t take this as fact and fight them how you want
On Ragnarok I had the 'best' surprise ever and encountered one not knowing what the heck was happening I was gathering meat from killing animals and noticed some dust following me I stayed to see what it was and a giant worm appeared and killed my pteranodon and me along with it
You find these things in the open part if the desert in Ragnarok. You really just need a good level Thyla and you should be fine. There a bit overrated, trust me I used to live in the desert. Mantises are tamed with Deathworm horns.
You can lie down prone on the ground and enter orbital camera to see them under the sand. But beware, there's a glitch where they can despawn as you approach, because of this glitch I've never gotten the chance to fight one