Sounds like a fun challenge you had there. I havent killed the Elder since the patch but about to and will see what happens. One thing that helps if you look around before summoning him, making sure there are no grey dwarf nests nearby. Also make sure you start the fight in the morning as at night alot of random enemies spawn in the black forest. Other than that i have no clue why you had an army spawn, hopefully i will find out soon. If i find ANYTHING UNUSUAL i'll report back.