The best way I've found to get wyvern milk is to lure a low level female poison wyvern, since their poison balls can't reach you an an argy, and pick up a friend, who is hopefully available, and just fly away and have the wyvern chase you. Have the friend constantly shoot the wyvern with some sort of tranq and watch it go down. You will need an argument with 200% speed to do so though... After this you should get 5 milk which is about enough to last for a couple of hours... just keep repeating this previews and you should be good to go
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.