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 and stage 17. So it is stage 6, 13, and 18.  

Rewards include: All purple heroes (ExokidArmored VillainArcane WarlockPyro PeteHead Hunter); all blue heroes (Battle BeetleShadowswordMechanoidBadboneOgre Chief,Crackshot); gold (19655), souls (362), rings (7), and Purple, Gold, or Coral Experience Cards (eggs). 

At level 80 you are awarded sets of 92412 gold, 1064 souls, and 28 jewels and/or rings.  

In a single solo campaign a level 80 player can earn approximately the following: 4000 mystic crystals, 500k cash, 15 blue heroes, 10 purple heroes, 28 rings, 28 jewels, and 2000 experience points.