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.
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by playing online mode you get HR points from completing online quests. which increases your rank.
You can get it from Gypceros.
Mining in the Tundra.
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you have to kill a piesloth. it is one of its carvings