I mentioned they're "working on" those OP spells, which mean a lot of these OP spells are still in concepts phase and Amagal have yet to actually make them work.
Amagal has small territory, which means their defense line lacks depth and can be easily overwhelmed. The wall of Manastyr is strong, but not invincible.
They're lacking manpower and resources as well. Magic couldn't create everything, and mages require decades of training so every casualty is a huge blow. This makes them vulnerable to war of attrition.
Imagine them as Germany in WW2. They're indeed OP if they can end the war quickly, but if the enemy survive the initial onslaught, they'll be slowly ground to dust in the long run.