So AFAIK your pierce effects your summons. For pretty much every class right now cows, plaguelands (+lich king if you can handle him), and pits are the best places to start farming. So keep it up
. Grats on being able to do Dreamlands. That being said, you won't kill Mappo with a summoner Necro. You need something that does cold, lightning, or poison damage and a lot of it. His resists to fire, physical, and magic are too high for you to pierce until way lategame.