Hey mate, Militarum tempestus are extremely specialised and due to them only really having like 4 or 5 units to play with.
My personal opinion is that you are better off allying with someone, eg, guard (astra militarum) as they have stuff which can fill in the gaps and weaknesses of a highly specialised force.
https://www.youtube.com/watch?v=h5j-H5ZM14c
Heres a link to a battlereport, where someone plays plain militarum tempestus, and i think he does well. valkyries seem to be the way to go.