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