I must say that your Chaos Altar seems a bit on the large side.
I used the Engine part of the old Squat Land Train for mine (on a temporary basis), as this seemed a more 'Altar sized' War Engine to me, and I found that it didn't draw as much fire as a result.
I also find it's not especially easy to take out a Chaos Altar, given that it is a War Engine attached to formation of non-War Engines, especially once you have summoned a Greater Daemon to that formation as well, as your opponent can end up having quite a few target priority problems. That said, beware of Warlord Titans on sustained fire, they are very nasty against this sort of formation.
I field a Khorne/Tzeentch list myself, so I cannot offer much of a commentary on your unit choices, also because I really don't like Slaanesh (well, you cannot expect an Eldar player to like Slaanesh, can you

)
_________________
Soñando con una playa donde brilla el sol, un arco iris ilumina el cielo, y el mar espejea iridescentemente.