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