When you want to tame a Rock Elemental don't always use cannons, they are expensive to build ammunition for and they don't always hit the rock elemental, plus you are in more danger of being hit by it since you're immobile whilst mounted on the cannon. Instead what I did was I used rockets and it was a breeze, I lured the rock elemental into the giant rib cage in south zone 1 and we started shooting it through the gaps. It also cannot hit you through the gaps 90% of the time and I only missed one rocket. If 5he rib cage is taken just look for a different one or try lure into something similar like a big cliff or something.
I got a level 150 lightning egg and I was so excited to hatch the egg...that when I went to put the egg down I accidentally ate it instead. :( ps4 users: R2 ISN'T GONNA DROP THE EGG YOU'RE GONNA EAT IT LIKE ANY OTHER EGG. REMEMBER THIS. LOGIC BEFORE EXCITMENT.