If you're getting killed by Desecrator fire, molten fire, plague pools and arcane lasers, chances are you are using the wrong skill. Switch to Deadly Reach + Keen Eye rune, and learn to Shift attack vs mobs from a few feet away. It's called Deadly reach for a reason, it has REACH. You will no longer be standing in places that kill you.
Also, save your 3 second invisibility for when the arcane laser is almost going to hit you. That way you don't have to stop attacking and move. If it's not available, and you can't walk out further to avoid the lasers, walk right through them in the direction they are coming from. It shouldn't hit you for more then half your life. When your on the other side of the laser, heal either by BOH or a potion. For desecrator fire or plague pools, if you happen to be standing in it, just stop attacking, take 1 step to the side, and shift attack some more. It's pretty easy. Try to never run away from mobs, that's when they tend to hurt you the most. Try to always be a few feet away. Never too close, never too far
Use earth ally, and always summon him so he's standing between you and the enemy. Since your shift attacking while standing still from a small distance, there will always be room for your meat shield to be between you. Trust me, this helps a lot ! When I figured this out, it made area's like the Keep a total joke. Just learn to position your ally right in the doorway, while your standing a few feet behind him shift attacking.
Of course, all this is based on you already having high enough resists, armor, and LOH to actually be able to tank a decent amount of damage. If you're super weak, nothing will save you. If your stats are pretty good, but are still dying, adopt my tips and you'll walk through areas where you used to get raped.