rathalos, idk, but the easiest way to get a lg monster bone is to kill a big monster...
I suggest Yian Kut-Ku.
Hes easy, i can kill him with only using 2 first aid med kits.
I got cyclone (the upgrade to the hunters knife) it is dual swords, and i got high metal
armor chest an vembraces for the gunner and blademaster because they hav high dragon
protection. Thouth its ugly, it does the job.

sorry to say.. but i cant really help u coz i dont play ths game.. and its boring.. dont
you get it..

You need kill rathalos it appear in reward for complete quest of '' attack of the rathalos
''

You need kill rathalos in quest called ''attack of the rathalso'' in ugrent quest 5 to get
rathalso shell and lg monster bone. And in this quest where you fight monoblos in ugprent
quest called ''horn of the monoblos''and to break his horn and get in reward. You can get
wyvrex marrow in reward in two quest called '' catch a rathalso and a troublesome pair''.
There are rare thing too...
I hope this help!

Sorry. I froget tell you about rathian's carve.
You fight rathian in quest called ''queen of the sand'' to get rathian shell in reward
make easy your best blade have good attack.
You do better it.