I recomend being HR3 and cutting off its tail. (you have a better chanc) Now for earth crystals just do a gathering mission for ore and u (usually) get alot.
Yep. I need 41 more Earth Crystals then I can make my Eternal Strife , I just go monoblos training and get lot's of Expand pickaxes and then comibne with earth Crystals and I get 3 - 4 at a time