Rathalos.. try bringing with you flash bombs, throw it in front of him, thunder elemental weapons, and if he flies, always go beneath his shadow and plant shock traps..
Diablos.. still, flash bombs are usefull, in the desert lure him where his horns will pinch a wall, ice elemental weapons..
gravios.. well just bang a hell out of him, in ur lvl there is still no fencing skill available so your slash will obviously bounce off the first time.. trying bring bombs.. shock trap him and plant large barrel bombs and ignite it with one small barrel bomb.. ater opening her chest portion, you can easily slash it of!