Its actually pretty easy, use knight, mage and princess.
First turn you attack with matilda to set the attacks on red. You shield red with himself and you use shock shield on him so his attack that deals 5*so much (i dont remember) damage will cause him 5*shock shield damage. Keep healing with princess, use knight to attack, then protect, then attack, etc. so the attacks will keep being focused on red. Use shock shield once in 3 three turns so the shield stays on and attack the rest of the turn. There ya go
