If you want to get max DPS time on the boss, even Posthaste + Glyph won't be enough to save you should he target you. In any case, you can control who he fixates, and it should always be on one of your healers anyway. I tend to use disengage offensively to catch up to the Thok by the time he reaches 15-17 stacks of Blood Frenzy. If you have a druid in your raid with as free symbiosis, you can get dash as well.