There are ways to do what you want, even if none is perfect.
This is the mathematically easy solution: if a boss has a robustness of 4 to 6 points above the norm, it will take a lot of testing to make a successful move to eliminate them. By giving them at the same time many things they can do per turn, you can keep a whole team "busy" with attacks / effects / other side tricks. This is how a dragon is written in the basic rules.
However, according to my experience and my opinion, bad option because it just increases the incredible swinginess of fighting Savage Worlds. Regardless of the robustness, there is a non-zero chance that the fight will be over in the first round, and on the contrary, there is a non-zero chance that it will take "forever", or at least longer than the player characters .
Since hurting the boss is essentially random, players will not feel very involved during the fight. Their decisions will not matter much, they will only wait for this triple explosion of damage, while hoping that the BBEB will not have it first.
Similarly, multiple actions from one source are more effective than the same number of actions from multiple sources. If the boss is shaken, all actions are lost at the same time. And especially in the case of multiple melee attacks, there is a serious problem of focus, in which a guy is touched by everything and dies immediately. Or conversely, in case your players stay wisely at hand, all these clever multiple options would become unusable.
Some examples of bestiary monsters use this approach, in that they can only be damaged / hurt by certain specific things. For example. a vampire can only be shaken by a result that is not sunny, sacred or an issue crossing the heart. This obviously makes the monster undefeated until weakness is found and exploited.
It's a little better than the great tenacity, but also a bad option my opinion. This is for two reasons:
Most players and GMs allow their controlled characters to fight until one side is dead. If, however, a monster is undefeated by normal means, then an encounter in combat with that monster is inevitable: Total Party Kill. As GM, you have to rethink the fight to set final states other than death, and really make sure your players realize that they have the opportunity to try to disengage. It is not easy.
This type of approach is fundamentally railway: what the player characters do is meaningless as long as they do not find and do not exploit weakness. Especially if the scenario to discover this weakness is poorly designed, it also kills all the pleasure of the players.
Each GM Wildcard has 2 Bennies per session, and in addition, GM has 1 Bennie per player around the table, which can be used at any time, even for extras.
One of the main uses of Bennies are the soaking rollers, which deny an injury. This means that, more than the Wound track, it's the amount of Bennies that equals the amount of HP's other games. Once a boss can no longer impregnate, it's usually quickly over because the injury-related penalties make it all harder and easier for their opponents.
Therefore, if you save the Bennies generic pool for the final showdown, the Boss becomes proportionally more difficult. With four players, he goes from soaking 2 times to 6 times, so the fight also lasts 3 times longer (on average).
Savage Worlds is a narrative system in that it only provides mechanisms and then lets the interpretation of these mechanisms into the hands of the GM. What does it look like to be shaken? What real action, if any, is represented by a Soak roll (or a Bennie expenditure in general)? This can be used to make a more dangerous enemy appear than his gross statistic block would suggest, simply by giving an unusual narrative explanation to the character's quench / recovery rolls.
For example, one of the most memorable fights of my recent campaign was that of a guy with rather average fighting stats. If the main fighter of the group had been there, it would have been finished quickly, because the only thing that was out of the ordinary was that the enemy did not know all the modifiers of the wound.
However, the players involved became really desperate during the fight, because I told this guy as being basically the relentless man. When the thief opened the fight by a saber attack called to the head, I described the successful roll Soak as (instead of, as usual, the injury is slight despite its initial appearance), continuing to function despite a gaping head injury and that there is nothing left. to see with. Later, he impregnated a wound of the ranger who was charging on an unseen mount and impaled it with a spear. The description was that the man had grabbed the spear and pulled it by pushing the rider and going up on the ground. None of this had a mechanical impact, but it made the fight extremely memorable and impressive.
How difficult it is to design a meaningful but difficult combat encounter in Savage Worlds, better The option is not to rely on combat, which fits better with the genres that Savage Worlds has tried to emulate. What makes Toht and Belloq so problematic is not that they can beat Indiana Jones in a fist fight, it's that they have a resource advantage with their Nazi support and a greater willingness for action. amoral.
Therefore, instead of trying to bump against a boss, players are supposed to lose it, which creates secondary conditions that make it difficult or undesirable just to attack or kill the villain. This can be as simple as not leaving them in direct contact with the heroes until the last act. A villain can be intimidating and difficult to defeat without any personal danger.
For example, in my current campaign, my group is currently facing a mage they've managed to easily defeat before. This time, they are much more cautious, not because their opponent has better statistics, but because the environment is different: the last time it was an isolated encounter in a place that did not hold very much. This time, he is a cult leader who has infiltrated local law enforcement and who lives in a place where any entanglement in the law of heroes could provoke a civil war. Therefore, even if they met this mage in the street, they could not treat him as easily as last time.
Although of course, not really answering the question, the best option is simply to accept that Savage Worlds does not perform very well in individual fights and duels. The system is designed for larger scale battles, with multiple participants on both sides.
Therefore, instead of trying to make this Big Beefy Boss work, it is better instead provide a host of adversaries, with different abilities, who work together, to give the team a difficult challenge. This also prevents the Boss's anticlimax from falling in one fell swoop due to a quadruple explosion.
This applies as much to combat as it is to combat. By abandoning the idea of a single central villain, the narrative breadth and responsiveness of your scenario to the player's actions increase dramatically. I will quote the beautiful article by Justin Alexander "Principles of RPG Villainy" for a more in-depth exploration of this topic.
And, because it should be mentioned because it comes from an official source:
In the "Daring Tales of Adventure" booklet, the authors introduce a special rule to use for more Pulp-type scenarios. Paraphrased because I only have the German translation:
A Wildcard GM can at any time in combat spend a Benny to run away, ie move his maximum speed and ignore any obstacles or secondhand attacks that prevent them from escaping. They automatically succeed in all the checks they need and can take this action even if it is not their turn in combat.
This specifically helps to reoffend the bad guys and prevents them from dying prematurely. But I would also call it a bad option, because the notion about a leader dying prematurely and then just inventing a rule to prevent who are the two railway symptoms.