Why not The Escalibur?
the same thing happens to me. i normally have my djinn on standby and then use my best summons against it until all djinn are recovering then i use my healing and defense psynergy until my attack djinn are set and use them against it and then use my defense and healing djinn until all are on standby. by now theres 2 heads and the 2nd is pretty weak so i use my 2nd strongest summons to defeat head #2 but by now my characters are super low on health and Jenna doesnt have enough set djinn to use cool aura and felix nor issac can use their heal moves cuz theyre low on psynergy and the dragon beats me with that Cruel Ruin move. can some one help me here?!