First, get some health and mana potions and anything that boosts your mana pool (+intellect gear). You may also want a wand with good DPS. The first stage is the hardest, as the Obsidian Sentinel hits very hard at first. It does purely physical damage, so use your Mana Shield to stay alive. Fight using only non-targeted spells and your wand. Use Frost Nova to give yourself time to recover mana with potions, mana gems or Evocation. Soon, the Sentinel will splinter and spawn an Obsidian Shard. At this point, if you're in danger, you can flee the instance and recover outside. Your "progress" so to speak will be saved.