You can't be invincible, but with the right armor and items you can get pretty close. You just need to try combinations. Being invincible would just be cheating though.
by playing online mode you get HR points from completing online quests. which increases your rank.
You can get it from Gypceros.
PLay-asia.com
Mining in the Tundra.
you have to kill a piesloth. it is one of its carvings