. you really have to manage his debuffs properly and trade off people tanking phoenix. you always need someone to tank an attack because otherwise he hits you with the full party AOE and that is really debilitating. there is a forest tile below phoenix that you should be using. phoenix will attack the person to the right of him first, then phoenix will attack the person underneath him. this battle is basically a dance of keeping your healer in until they need to back off and recover, and moving out your tank when they can't bear the load any more, then having someone tank phoenix in the forest tile while your tank recovers (maybe shifting off this burden between two people). boots are extremely helpful! boots will move your tank up to 4 mov so you can safely move them to the edge of phoenix's range, then they can avoid the AOE. it is of course paramount that you disable the adds first; you'll want your tank on the bottom spawn closest to phoenix so they can get into the forest tile immediately, and your person that takes care of adds next to them with the rest of your party above phoenix. keep the person that deals with the add in range of phoenix's AOE on the first turn; phoenix's AOE can apply molten armor and if they take that stack it will lessen the burden on the rest of your units.
i would say the most important thing in this fight is to not get greedy. you'll learn what you can tank with through experience, but any more than 2 molten armor stacks is risky business.
you have 15 turns, use them!
and it's important you keep other people than your tank and healer in the range of the AOE to mitigate the chances of your tank getting another stack of molten armor. 2 healers can be good for this fight