Unless there's a need to differentiate between different messages having different color codes, just do a text match for the text within:

Code:
on *:TEXT:* has entered the battle!*:#:{
  set %enemy $1
  /os client join %enemy #battledome
}


Unless the /os alias contains code to verify that the text is being done by the correct gamebot, you should have code at the beginning of your event handler to make sure it's not me sending that message smile

Something like:
Code:
if ($nick != NameOfGameBot) return