Bring around 6-7 or more rexs and when it lands everyone get around it and trap it. Deathworms will come and knock u back but just keep moving forward slowly so u dont get pushed out. Even when the rock golems come don't stop hitting it. Goodluck boys :)
When fighting the manticore a good strategy is bring in high level Rex's with good saddles and circle the manticore it then becomes pinned allowing you to kill it without it flying all around .... watch out for the poison it shoots at you though it will kill you off your mount like the poison wyvern ..
What I did is I bred up my lvl 450 trex switch some lvl 300’s (I play at max difficulty so it was easy to get them) then I waited until I got about 10 eggs and imprinted on he ones with the best stats. Then I brought my fire wyverns that I just got they were both lvl 35 and we’re good at dodging (they lived) I got two rocket launchers with a few homing missiles to fire as well along with my assult rifle I was in full riot armory as well and had an extra set handy as well. This is just what I did for the beta version good luck!