Here is the full list of all 25 Arena achievements.
Complete the tutorial
Win 25 online battles
Win 100 online battles
Win 1,000 online battles
Win 3,000 online battles
Win 6,000 online battles
Earn 80 glory points in online battles
Earn 500 glory points in online battles
Earn 1,000 glory points in online battles
Earn 1,500 glory points in online battles
Earn 1,800 glory points in online battles
Send 100 combat units to allies
Send 1000 combat units to allies
Send 5000 combat units to allies