I'm still stewing on how to properly make "Boss Battles" feel potentially brutal, but without killing everyone instantly.
I played the Dark Souls Board Game and I've been stewing on how to incorporate a bit of that "AoE you can see coming" but without doing it in card form and memorizing attack patterns. That said, I'm not sure how else you'd do it.
I've considered the Reflex Save "dodge roll" thing as a way to allow players more protection:
When fighting defensively, you may subtract 10 + Dex modifier from your AC, and add the remainder to your reflex save. You may take a reflex save to avoid hits your opponents land. If you are successful, you take half-damage. If you beat their total To-Hit roll by 5 or more, you ignore all the damage.
Something like that, because it allows your mediocre AC to have a chance of still dodging massive attacks.
I've also considered splitting the turn into smaller phases, allowing them to "see an attack coming" and move out of the way.
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That said, I'm looking for ideas. Maybe a "perception" or knowledge roll of some kind could let them see certain tells for the attacks it will probably do; allowing the party a certain prescience when rolling well, while giving them pause when they don't.
I did find the Dark Souls board game to be satisfying for the boss battles, and would hate to make a big impressive scene in the rpg less impressive than just playing a one-off board game.
[Note: I do realize that playing monsters "correctly" and in environments to which they're suited helps and I've been able to do some pretty cinematic things, but this particular "Avoid the big attack!" one is difficult to emulate.
Also, I read TheAngryGM so I've been kept up on his boss-battle implementation ideas; so this really is just about "big attack prediction" mechanics.]
Well I don't necessarily know how to deal with the prediction quality of things since being a turn-based system makes that more difficult, however when I do want my own boss creatures to have sweeping AOE attacks or big "you don't want to eat this" type things I'll usually provide the target of the attack or ability a perception or knowledge check to detect something bad is on its way (The new Knowledge: Martial from Path of War works well for melee monsters I find as a secondary option). They can choose to "Brace for Impact" by making a fortitude save to resist the damage for 1/2, gaining a bonus to the save equal to their shield bonus to AC, or they can attempt to dodge the attack by making a reflex save.
Heavily armored and shielded characters often have plenty of Hit Points and attempting to brace for the attack has a greater chance of success, while lighter armored dexterity warriors have a much better chance of just avoiding the damage altogether. I also use the rule that if someone is bracing for the impact they can defend others in the area by taking the hit for them, using their body as a shield against others who are too injured/debuffed to properly avoid things. The tanks of the group usually find this to be both cinematically enjoyable and still keeps them valued even if they choose to invest in defenses over murder power.