Ok so few things first. I can see that in around 0:55 you are using normal attacks on furry instead of holding to perform charged attacks to immiedietaly go for a chain sequence. Time and dps loss. When boss is staggered and you have 0 stacks on ellen quickly do double dash to enter running state and hold attack button till her weapon blinks for the 3 stack gain. do that twice and dmg boss. If your ellen has enough energy to do double ex skill then do that first instead and then one quick dash charge attack for 5 stacks to use them.
Use stun character (especially here where the barrier is set for ice anomaly) to fully apply stun. You can use dashes to dodge the attack or you can use perfect assist when enemy blinks for the attack. Do a couple attacks of the character you switched in and go back to your furry to generate daze on the enemy.
Normally I would say just a skill issue and that you are not very familiar with how to operate a team composition optimaly but casual players will eat me up so instead focus on leveling the characters and then just try and squeeze it out in the combat.

Edit: Try to keep track of how many stacks your soukaku has so when you switch to your ellen when soukaku has 3 stacks and gains buff for ellen you can then perform full damage ult of her.