What you are doing wrong is using Amplify Damage when you should be using Decrepify. Decrepify essentially stops Baal from casting ANYTHING, because his hit recovery is slowed down so much. I usually don't even teleport my fishymancers, as they don't need Enigma. I just run around to Baal, Decrepify him, then run past and start casting bone spear and re-casting Decrepify as needed. My merc almost never even gets hit, as Baal is slowed by the combined effects of Decrep, my clay golem, and a cold-firing necromage.
There are those who are wise, and those who are otherwise.