The monster I created is very powerful and would make a great boss fight or companion for your players. Feel free to use it in your campaigns when you want someone dead.

During it's creation I took a lot of inspiration from Mahoraga from JJK and used the Oni as the base.

https://www.dndbeyond.com/monsters/4230879-maledictus

Seems very oddly balanced. Definitely not a CR 30; any high level party could absolutely wipe the floor with this using a variety of methods, most likely in the first round. Granted, balancing monsters for such a high level of play is nearly impossible. the first thing I notice is that the listed stats are not properly reflected in the stat block. The ave DC should be 27, to hit should be 19, and damage should be 1d8+10. That being said, with 33 AC (there is no reason not to use Shield every turn), 200 speed, and a +39 to hit guaranteeing at least 85 damage, this monster, honestly, seems boring. This type of unavoidable damage is typically not published in monsters for good reason. May I ask what your idea is with making this monster? I could help you build a new one.

I try to avoid just making things hard by using big numbers. Never hitting isn't very fun, neither is seeing the enemy roll a 35 for their WIS save. It also makes only a single attack every round, which means the fight is generally going to feel like it's just standing there as everyone wails on it.

I like to make fights engaging with:

• Legendary Resistance (or something like it). You need to be able to avoid the "I win" spells. Let the lesser spells hit so spellcasters don't feel completely discouraged. Even when this shrugs off spells, there is a feeling of progress because you've expended a use and they're wearing down that defense.
• Actions, actions, actions. You should be acting at least three times a round. Lair actions or similar environmental effects can count as one. Legendary actions can fulfill this role, but for serious business BBEGs I just give them a whole extra turn. You need to balance the action economy to make it feel like a challenge and avoid a simple "surround & pound" scenario.
• Different stages. My big fights are really two or three fights. "This isn't even my final form" is used all the time in video games for good reason - it's badass, cinematic, and fun. Changing the environment between stages can make the battle more dynamic as well - smashing the ground so everyone falls into the chamber beneath, or completing the ritual to plunge the battle into another dimension.
• Distractions. Toss in some minions that are easily downed but provide support. Maybe they explode when they die, maybe they can be eaten by the BBEG as a bonus action to heal, maybe they leap in front of attacks to shield their master, maybe they just use the Help action. This is another way to eat up actions so that your boss doesn't get steamrolled in 1-2 rounds.