Here is the full list of all 20 Thunderflash achievements.
Destroy 500 enemies in "Survival" mode
Destroy 250 enemies in "Survival" mode
Beat the "Boss Rush" mode
Beat the game without using "Stage Select" mode
Beat the game in Hardcore Mode
Beat the game
Complete the Mission 5
Complete 5-4
Complete the Mission 4
Destroy 5 cannons
Complete 4-2
Complete the Mission 3
Complete 3-2
Destroy 100 enemies in "Survival" mode
Complete the Mission 2
Complete 2-3
Destroy 3 towers
Complete the Mission 1
Destroy a Helicopter
Destroy a Tank