Campaign Clash is a solo campaign where your army faces an opposing army (similar to the heroes arena).  There are 20 total stages and rewards are given at stage 2, 4, 7, 9, 11 (5 rewards)

Each hero gains 337 EXP if leaving the campaign after Level 12, 418 after stage 14.  

Mystic Shop appears once you've cleared stage 5 and again after stage 12. 

Rewards include: All purple heroes (Exokid, Armored Villain, Arcane Warlock, Pyro Pete, Head Hunter); all blue heroes (Battle Beetle, Shadowsword, Mechanoid, Badbone, Ogre Chief, Crackshot); gold (19655), souls (362), rings (7), and Purple or Gold Experience Cards (eggs).