Here is the full list of all 21 GUNS UP! achievements.
Reach level 25
Be in an Alliance that receives a badge at the end of an Alliance Battle Season
Obtain a Legendary card from PVP rewards or card pack
Play one more than your daily Alliance Battle limit
Earn 300 Alliance Points in a Season
Open an Epic Card Pack
Win an attack against a nightmare opponent (red skulls)
Spawn 5 different Veteran unit types in battle
Lat 6 minutes in CPU Defend
Earn 4 or more Veterans in a single battle
Fully rank up 3 units
Obtain a Level 7 Wall
Use 50 attack cards
Win 25 PVP attacks
Collect 3 daily rations
Attack 3 friends
Win 10 attacks in a row
Assign 2 perks to 1 Unit
Attack and get 10 Monster kills or better
Customize your troops
Complete the tutorial