Gonna Go When The Volcano Blows

I need this Achievement and I fear I have one week to do it in. So, I want a full proof (and possibly underhanded) means of getting it. We're not saved to the raid until Sarth is down, right? So, if I screw up, I can just leave the raid group and hearth? However, basically I can just dot Sarth and Hellfire to death.

Though, I also read that if you swim in the lava, you don't get struck at all. Is this true? I'd like some confirmation, 'cuz I'd gladly DPS in the lava.