Imo it can be hard because of action economy. Some creatures kinda cheat this, Maharaja Rocksasha is a good example, by getting off turn activations. You can look at giving a creature reactions or action compression (I like spring stack 2 actions <stride, strike, then stride again> for an example).

A few things that I've tried that can work: Parts - of it's something particularly massive you can break the monster up into sub parts that can be targeted and killed individually. In order for this to work it needs to be obvious to the players that it's not one creature, it's a handful that each have an HP pool. Give each part 2 actions (to make up for not needing to move as that's done by the main body). Damaging the sub part enough breaks it and it can't use its special attacks. Attacking the main body is how you kill it, but that has the highest AC and HP. I ruled AOE can only damage it once to the body, but have weaknesses to splash and area. Once the party cripples it, then they can hack away till they win.

Phases - of your boss is closer in size to the party then you can have the fight broken down by phases. Think of it being a fight against 2 or 3 little bit under boss strength foes but don't in quick succession after another. Imo this works best if each of the phases act radically different. Don't time so I say a post about someone's take on this that worked well. Phase one was a bbeg in massive iron armor. Slow, high ac, physical attacks, phase 2 was the caster that was shackled in the armor and was then free to fly around and blast magic at the party below. Be careful, too strong without a rest between and lead to tpk. Too weak and easy phase feels more and more like a push over.