[The CEE]

Here's a Valkrill themed army that immediately came to mind:

240 Nhah Scirh Cultists x3
260 Mezzodemon Warmongers x4
500 points/20 hexes

The Cultists and the Mezzodemons should do a good job covering each other's weaknesses. Does the enemy have a special attacker to bypass your exoskeleton markers? Send in the Cultists. Does the enemy have swarms of common units? Send in the Mezzodemons.