Played with 4 characters:
*Marksman (pure) - with Fire arrows
*Knight (pure 2H) - with Attack of opportunity + Whirlwind + dmg stance +Battering Ram +Cure Wounds ++Thick Skin (more Tanky)
*Aero+Hydro+WC+Pyro (let's call him Archmage) - with Ice Shard+Oath of Desecration+Wildfire+Teleportation ++Glass cannon
*Jahan - Rain+Blitz Bolt+Teleportation (I think that's the name of the lightning attack)
Rushed in with my Knight and start the chat with Rex, meanwhile scatter my other troops like this:
*Marksman- near the stairs of the Lighthouse boss.
*Archmage- behind the Knight (10m+- so if someone cast burning/poison he will not get in the way).
*Jahan- near the Archmage but closer to the Baron boss.
When I was ready I finish the chat with Rex and start the fight.
Because I had high leadership = +initiative (don't remember the number but it was high enough so all my chars was first)
With my Marksman I fired the fire arrow and ignite the entire place of the lighthouse boos (fire dmg really hurt him), he will start to walk and get more fire dmg.
With my Archmage I boosted my Knight (both haste and dmg).
Jhan use Rain on Rex and Fire lord and TP the baron on them.
Knight use stance + whirlwind (1/3 of HP down for 3 of them)
Rex use some magic and start to run awasy ----> Attack of opportunity deal more dmg.
Fire lord scorch my Archmage (Heal if you want).
Lighthouse start to walk and spray ooze on my Marksman.
Baron use Inspire and attack my knight.
Marksman continue to fire on the lighthouse (you can stun if you want if you got stunning arrow).
Knight use Battering Ram on Rex+Baron+Fire lord and deal more dmg + potion or Cure Wounds if needed.
Archmage use Ice Shard on Fire lord -----> 150-200 dmg easy, ice is the fastest why to kill him. because of Glass cannon you have double AP recovery...so you can TP again and cause more dmg..or other spell.
Basically that what I did, worked on 2nd try after I knew that he summom.
I hope it helped..even a little
I used rain because i want to create a puddle and electrode them...don't remember if they can be stunned.