A summoner can definitely do baal runs solo, and he can do them fairly easily if you have Enigma to help you get to throne room. In fact, my summoner doesn't even need to summon any skellies to beat minions/baal. He can tele straight to throne and merc and golem take care of minions with the assistance of amp damage and maxed corpse explosion. Very easily and quickly I might add. Baal is also easy with a crushing blow merc and a clay golem or iron golem made from an item that slows enemies (my personal favorite, Kelpie Snare fuscina @75% slow). With no skeletons, mages, or revives summoned I still kill Baal in less than 1 minute. With all 15 of his skellies and 15 mages and venom lord revives he kills Baal in around 10-20 seconds. I do admit my necro has good gear now (Beast zerker and merc's Infinity cryptic axe and 6 BO Cta), but he used to kill reasonably quickly before he had these items as well. 8 player games slow down the killing power of a summoner slightly but they slow everyone down so no big surprise there. A good summoner should fear nothing but the most overpowered and stupid curse Blizzard ever thought up, Iron Maiden (honestly how can you justify a curse that can kill a lvl 99 character with godly gear in normal difficulty in 1 hit?) Have fun and I hope everything works out for your necro.