The Ryleh dragons on crucible level 14 and Mappo will require close to 200 if not more to kill in a reasonable amount of time. There is no maximum amount of pierce that you can have. However, monster resist can go to a minimum of -100. Mappo has 200 physical resist and at least 200 fire resist. It would take 300 pierce to those elements to get him to -100 resistance and 100+ to even do damage to him. Note that Mappos fire resist appears to be higher than 200 as my fire trapsin with lk boots, (-10-15) 2 tricksters, (-30) and azmo armor (-75) couldnt do damage to him.