Hey guys,

for an upcoming game night, my players' party will come up against a mob boss that I want to have some oompf, so I'm thinking about making him a Legendary creature. I'm using the blackguard stats, the party is lvl 6 consisting of a sorcerer, paladin, fighter and rogue. I don't want to make it too powerful but I want the fight to be memorable, not a simple "whack 4 times, whack 1 time, whack 4 times, whack 1 time" kinda deal, hence I want to make it more fluid by making him Legendary.

How would you suggest I'd do this? Any tips would be welcome :)

I would give him legendary actions but not legendary resistances. Perhaps one of his legendary actions could be to summon up infernal power to get an instant save with advantage against certain conditions like stunned, paralyzed or charmed. Legendary resistance feel somtimes (to me) like the DM being arbitrary and their use can suck for the players, which is memorable in a bad way. 

The legendary actions could be just extra attacks, moves, spell casts and so on. Perhaps the blackguard also has been bestowed with the power to summon demons, devils or evil fey like certain beings from the lower planes can... It should be a percentile roll to succeed to add tension.

The thing that makes a creature legendary is legendary resistance and/or legendary actions.

So for your blackguard, you can:

1. Give it 1-3/day legendary resistance
2. Give it 2-3/round legendary actions such as:
   • An attack (1 action)
   • An AoE attack (2-3 actions)
   • Cast a spell (1 action)
   • Free movement (1 action)
   • Non-damage action like search or shove (1 action)
   • A buff or healing action (2-3 actions)
   • Summon ally (3 actions)
   • Cast a specific spell without slot (2 actions)
3. Give it a name and race, and include racial bonuses into its features
4. Give it a magic item
5. Give it new actions (could be legendary actions) that it doesnt usually have.

First, there's an article by Mike Shea about creating Legendary creatures: here.

That said, you could give them legendary resistances, and lair actions - but I think your main ask here is that you want "the fight to be memorable", and to make the combat "more fluid" yes? Don't forget there are many ways to make the combat more interesting and memorable without adding to the bad guy's abilities. There really should be an in story reason for an opponent to be Legendary - but any combat can be made more interesting and complex.

1. Add stakes to the combat: Don't make it just a slug-fest, put in goal(s) inside the combat, so that the point of the encounter isn't only killing your opponents, but killing your opponents might be part of achieving the larger goal: the heroes have to fight their way across the castle to the dungeon to stop the execution; the bad guy is trying to get away with the McGuffin through the city streets to the escape boat; the ship is sinking, and neither side can escape out of the hold because they're pinning each other down in combat; the hero is trying to drop a proton torpedo down the reactor vent port before the bad guys close in behind him and shoot him down.
2. Mix up the opponents: Powerful bad guys have minions, add low level, easily killed, mobile minions to keep the Party pinned down, give the bad guy time to escape, slow them down so they might miss that execution. Don't give minion bad guys a lot of HP, or super powerful attacks, or that starts to build up the combat CR too much. These guys are really movie extras to deflect the action in interesting ways, not to provide (much) of a serious combat threat. Think of the storm-troopers chasing the heroes through the Death Star in the original Star Wars movie.
3. Mix up the tactics: But those minions can really add complications to the battle! Add long range archers, add flying creature, add burrowing creatures. Have a mix of abilities, attack styles,  and battlefield complications. As a DM, maybe meta-game here a bit: wanted to add some flying creatures as the main "muscle"? Well ... the Ranger will be OK, but maybe add some low-level ground based minion attackers which add a really annoying complication to the battle, in order to keep the Barbarian happy & useful as well. Have the bad guys use their abilities together, and combine them in interesting and effective ways.
4. Mix up the Heroes: Hey, who says only bad guys can have minions? The plucky archer NPC laying down covering bow fire as the Rogue makes a dash across the great hall; the gruff Dwarven mercenaries charging into the enemy flank when the battle looks hopeless lost, etc.
5. Make the environment part of the story: The setting is almost another side of the battle. Combat is an open field is one thing. Combat on a series of rope bridges between giant trees in the Elven forest is another. Combat racing along precarious rock bridges in the volcano, as lava randomly fountains around, yet another. Hazards and dangers add a lot as well. Maybe you want to avoid the area with poison gas. Maybe it's worth charging through the geothermal steam and taking 1d6 fire damage, to be able to flank the enemy. Don't forget the environment can be dynamic as well: the bad guys cut one of the rope bridges, cutting off a line of movement on the battlefield; the giant pushes a tree over the ravine creating a new bridge; the building catches on fire.
6. Make the environment work differently for the bad guys: This is a subtle, but powerful one. The swamp creatures don't treat the bog as rough terrain. The Kobolds have a series of tunnels they can use to short-cut through the cave complex, where the heroes can follow. The Party can scamper under the thick canopy of low trees, but the Hill giant can't see or follow them. Battles can be really interesting when the opponents are really fighting on different, but connected, maps.
7. Don't forget, tactics change: Have the bad guys adapt their tactics. The wizard threw down a fireball? Maybe the skirmishes fan out. The Ranger is taking out the flying  creatures? Maybe the horse cavalry makes a charge to try and take her down. And don't forget to take advantage of sudden opportunities. Someone goes unconscious? Maybe the bad guy will take them hostage, and force the Party to stand down.

All that said, if you want the effects of Legendary Actions, Lair Actions, and Legendary Resistances - without stretching credulity by introducing an actual legendary opponent they can kill at 6th level ( some legend :P ) - you can simulate them with magic items and setting up the environment. Maybe the bad guy has a Ring of Fire Resistance which only has 3 charges, rather than a Legendary Resistance  to fire. Maybe the bad guy can light the oil pool on fire, rather than having an actual mystical Lair Action, etc.

Best of luck with all this - setting up, and playing out, cool combats can be a lot of fun :)

Wow, thanks for the helpful tips so far! To give you guys some more information;

The mob boss is a Tielfing, a descendant of Levistus (with appropriate racials). Initially he profiled himself as a somewhat jovial if not stern fatherly figure. When the paladin used Divine Sense, they noted that there was a fiendish figure near him, invisible. This is an imp that is holding his weapon, a glaive, near him at all times. He is somewhat overweight due to the luxuries he allows himself and due to his Levistian heritage he is a light shade of blue. Though he prefers the company of other Tieflings, he has various others that keep him company.

I'm using the stats of a blackguard since though he has some charm to him, he's more of a brute. At one point, a long time ago, the players were in the same city and thwarted one of his schemes. Afterwards, before they met him face to face, the captain that was responsible for the plan which the players had thwarted, met them again. He was visibly assaulted by Ellimber for his failure, showing the brutality of the mob boss. This captain turned out to be a traitor working both sides of the gang war and put the players on the path to stop both gangs before one of them gained enough power.

A couple of sessions later, the players had stopped an important shipment to the organisation, which led the mob boss to doubt his captain's loyalty. And after a slip-of-the-tongue by one of the players, he singlehandededly the imprisoned, chained, near-dead captain while the players could only watch. Since at the time they were down one party member which was petrified at the time, they could only stand by and watch.

I know for sure that the eventual battle with him will be memorable from a story standpoint, but I really want the fight itself not to disappoint. If this turns out into a single round of combat whakathon, it would be a major let-down.