Scyllas are absurdly easy to take down because they are made wholly from one giant Achilles heel. Strip that one part of Scylla of armor (say, one of its legs) and just shoot there until dead. Their size, instead of making them more formidable, makes them more vulnerable. No need to aim, you will always hit something.
Facing a Scylla never feels like a boss fight - she is a giant waiting to be shot.
How about making an exception for the poor Scylla, and requiring first doing enough damage to her to expose a vital organ, and then destroying that organ to bring her down? So once a limb is disabled, it’s not possible to damage her attacking that same limb.
It just shouldn’t be possible to take a Scylla down in a single turn (and yes, there are quite a few ways of doing it with one soldier).