The Big Guana, unlike the lesser regular Guanas, is not a pushover. He has Fire Breath which will dish out a few hundred damage to all of your characters (350-350 to the front row, and 100 or so to the back) as well as Power Crush which can hit one character for over 1,000 damage. It's also notable that often times the Guanas will use Powera or Faster on the Big Guana, rather then on themselves, making him even more of a threat. Assuming there are no Killer Birds in the battle this guy should be your target. Hit him with a couple of Aquaras before he can do too much damage.