Scenarios are unlocked after finishing the tier one campaign. The thirteen campaign levels have a scenario setting and 6 gold trophies appear on the map for further scenarios. Scenarios allow you to modify how the level is played and have ten different options each with three levels of difficulty.
Red tier
Enemies drop less essence 15%,30%,45%
Trap upgrades are more expensive 20%,40%,60%
Your base starts with less hitpoints 30%,60%,90%
Purple tier
Enemies are faster 10%,20%,30%
Your hero gains less mana/rage/focus 15%,30%,45%
Traps deal less damage 10%,20%,30%
Blue tier
Enemies have more hitpoints 15%,30%,45%
potions are less effective 25%50%75%
Enemies have a chance to respawn 15%,30%,45%
Each color tier offers a reward. Red rewards more gold, purple gives better Item drops and blue gives more exp. Each star activated = 1 difficulty with a counter being active on the side showing you both the difficulty and what your reward bonuses are. This means a total of 20 stars need to be on for this achievement.
Which twenty you choose I would say is down to play style and which suits your character better. I did this co-op with a friend and we were both mercenary's lvl 28 and 32 and we used these
Red tier
Enemies drop less essence 45%
Trap upgrades are more expensive 60%
Your base starts with less hitpoints 30%
Purple tier
Enemies are faster 30%
Your hero gains less mana/rage/focus 45%
Traps deal less damage 30%
Blue tier
Enemies have more hitpoints 15%
potions are less effective 75%
We had no difficulty completing it with these settings.