I generally don't use Mephrit unless I'm running a small detachment of just Deathmarks. Otherwise I find running Mephrit as a bit of a trap. Assuming the core of your army is 30 Immortals with Tesla, you want to keep at Max range.
I usually run Nihilak for the mobility and deepstrike stratagem. If I'm running Doomsday Arks or Doom Scythes I run the Storm Lords trait (can't recall name of it right) for being to move and shoot Doomscythes, free time extra "My will be done" and CP cycling.
In most lists I always have some scarabs and Wraiths to help screen and get out of melee. This is key right now.