Here is the full list of all 20 Redd's Runaway achievements.
Complete the mission 1
Complete the mission 2
Complete the mission 3
Complete the mission 4
Complete the mission 5
Complete the mission 6
Beat the game in Arcade mode
Survive more than 10 minutes in Endless mode
Beat the game in Hard mode
Obtain your first spell upgrade
Upgrade all your spells to the maximum level
Get one of Redd's alternate costumes
Collect all Redd's alternate costumes
Defeat all the bosses in Boss Rush mode
Neutralize 100 creatures (added at Results Screen)
Neutralize 1,000 creatures (added at Results Screen)
Neutralize 5,000 creatures (added at Results Screen)
Take 100 power-ups (added at Results Screen)
Get 50 lives (added at Results Screen)
Collect all the game achievements