Well, the specifications of an Elite Battle are :
Always a Cruiser Clash.
Each Elite Rank raise the Enemy fleet point by 1%.
Each 10th battle is a "boss battle" featuring a special unit. For now, we have a Space Hulk Ork, The Chaos Planet Killer, and the Chaos activated Blackstone Fortress.
Last but not least, and it might explain why it is challenging, all battles are automatically set on the heroic difficulty, which gives the enemy even more fleet points, and increase the Fleet points gains of 1% for each rank.