Posted May 20, 2013
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Here is a guide about it:
http://bbtactics.com/cage-breaking/
A rough tactic is: Blitz one of the pillars of the cage to get him down or at least pushed away, then get as many players at the ball carrier as possible hoping that you can take him down next turn.
And I recommend having a look at this instead of reading the manual:
http://www.games-workshop.com/MEDIA_CustomProductCatalog/m750496a_BB_Competition_Rules_Pack.pdf
Then try a beginner-friendly team such as humans or Orcs and have fun!