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