Umbra is the last boss of the game. This fight takes place directly after the battle with Nox - if you win that one, of course. Umbra fights alongside two dragons. Both she and her friends are dark anomies, hence light and fire attacks are the most effective against them. They can cancel your turns' cooldown and are able to counterattack your magic strikes. The boss can heal herself and her allies as well as block your ability to break their attacks. She can also provide shields and increase the damage dealt by her and her minions.
The first choice for this fight are Rubella and Oengus. She can create shields and Oengus can use the Taunt skill to focus all the attacks on himself. This allows you to quickly swap Rubella for Aurora. The protagonist can deal light damage to all enemies. There's no way the enemy attacks will prevent her from doing that if Oengus uses Taunt. Remember to keep replenishing Oengus' health with potions or with Igniculus' skills.
In case your warriors die, swap them with Robert and Finn. The first one is able to slow down attack cooldowns of your enemies, which will allow Finn to attack with fire. Such combination should allow you to quickly get rid of the enemy. Don't worry if any of your characters die - there's no point resurrecting them. Better swap them for another one and quickly attack.