It's difficult to design an encounter for a party that we don't know. In addition, what's the goal of the encounter and how does it fit in to the overall narrative? This is not the BBEG fight, so what's the goal? Wear people down? Give them a taste for how difficult the final boss fight well be?
My general advice is kind of a a repeat from another post, but here goes
My general advice is kind of a a repeat from another post, but here goes
- Monsters have advantage on a regular basis. I justify it multiple ways, or just secretly roll it. In this case I would give your dracolich "ice sight" where he can see through an icy fog that is obscuring for other people.
- Do more damage. Not just more damage, but more types of damage. Particularly useful for my wife's half-orc character that has resistance to weapon damage. The suggestion of necrotic damage is great here. They're going to be expecting cold attacks so mix it up some.
- Attack more. Bonus actions or just multiple attacks. Use lair actions to do damage.
- Attacks that do more than damage. Taking people out of the fight is boring, but restraining them by having ice grow around them that is slowly crushing them? Go for it.
- Transformations. This one is a little trickier (and to do justice would take more time than I have) but instead of making monsters big bags of HP, have them change at certain points. At a 2/3 of their HP they lose some AC but start blindly flailing getting more attacks at a bonus. At 1/3 their HP their acidic/demonic blood is spraying everywhere causing automatic damage (or constitution save for half). When they die they explode.
- Spiritual allies. Instead of giving a monster minions that can be killed, they send out spiritual allies (inspired by a cleric's spiritual weapon) that can attack as a bonus action. In this case it would be ice-devils of course.
- Counterspell. This one can be annoying if overused, but give the enemy casters counterspell. If you really want to be mean, the enemy caster is a sorcerer that can cast without verbal or somatic so their counter can't be countered.
- Ranged attacks. Almost all creatures have effective range attacks or can get to people that are flying away. I'd give the dracolich the ability to form ice-spears that he can chuck at people at will.
- Different saves. I make up a lot of monsters. So sometimes it's going to be strength saves, sometimes it's going to be intelligence and so on. I don't pick on individuals, but I do try to vary what I'm targeting.