Bastiodon is a fossil pokemon I got few by luck when excavating rock or you can just do reload save tricks.
you can use my trick one mon with focus sash and endeavour make sure that Mon is least level 1 that has endevour move you can use common candy and 5 level 45 Mon with quick attack or extreme speed even sucker punch to finish off garchomp by that time
I always have mon with perish song , quick attack, endeavour and trick room in pc if something doesn't go my way always work.
The lower accuracy strategy is by luck if garchomp miss it attack after doing it 6 times it less threatening since you know 90% move will miss.
A beast (Kricketot) You just need focus sash and 50000 money for rare candy if you have money bag password ON should be easy since you just need sell item you got to get 50000 very long grinding .
I had 6 Mon with pickup ability I got 3 free rare candy while grind,Kricketot this little guy here you got before first badge are solaris worst nightmare rise it to level 50 and lower it level back to 45 after you get the move perish song and you only lost 3 mon in the process.