In my opinion the fight against Shackler has multiple problems:
a) it is tedious but not challenging, shades are easy to kill
b) there is no indicator for the correct kill order
c) whether a guess is correct or not is revealed after you kill all the shades. This means there are 120 possible combinations to guess from.
d) the enemy can have access to revive spells
Currently I just power through the fight by repeatedly killing everything and waiting for the maximum resurrection count to kick in.
I would suggest the following changes:
a) Punish repeatedly killing shades in the same order for example by increasing their stats or doing damage to your team
b) Revive the shades as soon as a wrong guess was made. This drops the number of total combinations to 14