I didn't do any testings, but if you're gonna put only 3 Obsidian towers, and the mark is Darkness, why not throw the whole 3 out? But then... I don't know what to put...
-3 Obsidian Pillars
+2 Pests and +1 DeathStalker/Deadly poison?
BTW I'm not sure, and it's just the first thought I have seeing the deck,