1) Do blocks of color,
you can use an old illustration you did or a photo ref.
2)Draw the perspective lines and major blocks,
get a feeling for the space.
3)On top of it start painting and define the overall,
don't focus on a tiny area.
4)Start zooming in and polishing, or poping elements,
is a good idea to use some soft edge brushes
in combination with hard edge ones.
5)On a new layer draw the silhouette of the mecha,
and charcaters and repeat steps 1-4 on them.
6)Use some masking selection and gradients to pop things more,
maybe play with the brightness and contrast too.
Use filters to taste, crop and save :-D