Battle Islands
14 Achievements
1,000
60-80h
Xbox One
Xbox Series
Warmonger II
Attain 100 offensive victories against the enemy
100
1.88%
How to unlock the Warmonger II achievement in Battle Islands - Definitive Guide
So this one gets a bit harder later down the line when you start collecting a ton of tags but if you want to you can manipulate how many tags you have. If you go to a battle and wait past the 20 second countdown and retreat you will lose tags. the lower your tags the easier invading becomes.
This will come easily over time. A victory counts when you attack another player, getting at least 1 star in the process.