On both tactician and normal this was my main goal.
First i use the fastest character, with haste and speedbuffs to rush to Dame Aylin and they usually arrive within 2 turns.
Then I use wall of fire or Cloud of Daggers ( two of the best spells imo ) on the platform and bomb Ketheric with spells and/or martials.
when he goes Apostle, I used wall of fire, over the apostle, and then use high damage spells or attacks.
The gist of the advice in this thread is that one needs a character with high mobility for Aylin, a character with AOE for the big boss and the lesser grunts, and a heavy hitter/tank who can do a lot of damage and take a lot of damage to focus on the front lines. Then a fourth that is either AOE or high damager for extra security.
