We've all heard the problems with big, solo boss monsters in D&D. They can easily become boring, grindy, and non-threatening. The poor things end up getting killed before they get a chance to do anything cool. The PC's have access to a huge array of bullshit that can easy grapple, restrain, stun, or put your hapless boss to sleep. The action economy is fundamentally against a solo boss, which it makes it really hard to make a fun, challenging solo boss in the "Combat as Sport" style of play.
Here is a random, simple idea for helping with these issues: Scrap the action economy. Just throw it in the trash. Instead, do this.
After each player's turn, the boss takes a turn.
- Claw. +14 to hit, reach 5 ft., one target. Hit: 15 (2d6 + 8) slashing damage.
- Careful shot. The Archer aims at a target and slowly draws back his bow.
- The Archer fires. +12 to hit, 2d10 damage.
- Blood Moon. The Wolf rears back and howls. The sound makes your head pulse with wild pain.
- The moon turns blood red. The area 10 feet around the wolf begins to squirm with black liquid.
- Burning blood rains from the sky. Everyone within 10 feet of the wolf takes 3d10 damage.
- Grapple. The plant grabs the nearest player. Make a DC 20 strength save - if they fail, they are grappled and take d6 damage.
- The plant pulls the target into its maw. They are trapped inside and take d8 damage a round until they can free themselves with a successful DC 15 Strength check.
- Giant Swing. The warrior moves and swings the gigantic axe at the closest PC. +10 to hit, 2d8 damage. The axe is buried in the wall or floor.
- The Warrior struggles, then pulls their axe free with a crunch and holds it high above their head in triumph.
- Charge. The Boar aims at a spot with the most PC's clustered together in a group. It prepares to charge.
- The Boar charges in a straight line until it hits a wall or solid object, dealing 2d12 damage to everyone hit. Whatever object or wall it hit will be destroyed.
- The Boar staggers around, dazed. All attacks have advantage against it.
- The Boar shakes itself and regains its composure.
- Death ray. The machine opens up its chest and fires a death ray from its heart. 3d8 damage in a 60ft cone, 19 Dex save for half damage.
- The machine overheats! Its beating heart is exposed! Any attack that targets the heart double damage this turn.
- The machine closes up the iron protecting the heart and stops being vulnerable
- ✸ Desperate Flame. The dragon lets out a shriek of terrible rage. Ash and smoke billows around it.
- A cloud of ash surges from the dragon. The area within 80 feet is covered in smoke, making it difficult to see. The dragon inhales. The heat intensifies.
- All terrain within 80 feet is ignited like tinder. Each creature on the ground must take d6 fire damage on each of their turns for the next minute.
- A zombie latches onto you and tries to bite you. +5, d8 damage, save or become infected.
- Rotting hands grasp at you from all angles. DC 18 STR save or be grappled.
- The hoard drags you away from the rest of the party and into the mass of undead. 2d8 damage each round until you make a DC 18 STR save or another party member saves you.
Converting 5e bosses to this system.
Movement: Divide the movement rate by 4 (roughly - round up if needed). The boss can move that far each time it takes a turn. So instead of moving 40 feet a round, the monster could move 10 feet after each player's turn.
- Normal Attacks: You'll want to reduce the damage. If the boss normally makes 1 attack per round, I would roughly divide the damage by 4 to make it work for this system.
- Multi-Attacks: If the boss has a multi-attack (like Claw, Claw, Bite), then each component of that multi attack can just be a single action.
- Big Attacks: For a big attack like a breath weapon, an AOE effect, put it into an action chain. Try 1 or both of following:
- Add 1-2 actions beforehand to foreshadow the attack.
- Add 1-2 actions where the enemy is vulnerable afterwards.
- Specific Attacks: For something specific or niche, consider making it a reaction to a specific trigger. When the dragon is attacked from behind, it uses its tail attack. This is cool because it allows the PC's to learn the boss's behaviour (better stay away from that tail).
- Status effects: For big status like paralyzation or charm, put it in an action chain of 2-3 steps. The effect gets worse each step until it finally accomplishes the full effect.