For this you'll have to play as the Dwarves or Orcs.
I found it to be the easiest playing as dwarves, as those have artillery. Use the underway stance with your army and place it as close as possible next to the enemy army. When it's the AI's turn, and tries to get past you, you'll get an interception. Play the battle and you'll be in the underway. Do this 5 times during the same Grand Campaign and you'll get the achievement.
Even if all Beastmen armies are destroyed, they keep respawning after a certain number of turns. But it came naturally during my Dwarves Grand Campaign.