Beholder

Ally unit 

A good card to be used when in the Arena. Do not reveal it before your opponent hits it coincidentally.

Combos 

Best used against mass damage units such as Witch, Mage or Wyvern. Use the Griffin to deal great amount of damage or even finish them.

Enemy unit 

Never hit the Beholder with the Riffleman due to his low health. Best countered with the Shaman auto healing ability. Be careful if you have the Black Widow because the reflected damage on your units is poisonous as well.

Exploits 

Hit the Beholder with your Storm card if this will kill your card. Do it at the beginning of a new round and the enemy team will be stunned.

Hit the Beholder with any card if you have the Templar card. The reflected damage will activate Templar's shield. Best used at the end of the battle when only a few cards are left to make your Templar invincible.