Boss battles in the Dead or Alive series are often the final fight in story mode, although who the "boss" is, their gimmicks, and who fights them varies.
The original Dead or Alive boss was Raidou, a brutal ninja who was responsible for crippling Hayate, raping Kasumi's mother and bringing Ayane into the world. Besides the difficulty of fighting him however, there is no true gimmick that Raidou relies on to make the fight any more difficult.
In Dead or Alive 2, the Tengu, Bankotsubo, is the final boss who fights the player's character atop the Sacred Miyama. His fight is difficult since he uses a variety of gimmicks; he's able to fly towards the player to close the gap for a throw, he's able to use his wind power to attack the player from range, and in the original Dead or Alive 2, causes the screen to appear blurry whenever he is not taking damage, which can be disorientating for the player. In Dead or Alive 2 Ultimate, he also gains a special ability that allows him to change the weather of the Miyama, from Spring, Summer, Fall, and even Winter, which also briefly makes the screen bright enough to disorient unprepared players.
In Dead or Alive 3, Genra is the boss fight, and you fight him in his Omega form at hellfire. His gimmicks are similar to Tengu where he relies on zoning to make it very tough to approach him, using his tokkosho, ground techniques, and ninpo to keep the player locked out. Trying to throw him will lower the player's own health and knocking him down will create a fiery shockwave that can also damage the player if not blocked. The entire battle is also fought at an obscure, over-the-shoulder camera angle instead of the regular one, and the flames of the stage also blur the screen similarly to in Dead or Alive 2's boss battle. Like Bankotsubo, however, he is quite slow and can be rushed up close although his counters and zoning can quickly reset the gap.
In Dead or Alive Dimensions, all the bosses can be encountered in the arcade routes, including Shiden, Kasumi and Hayate's father. While he uses an altered move set of Hayate, his AI excels at countering and being able punish, so caution is advised.
Dead or Alive 4 introduces the notorious Alpha-152 as the final boss, a superhuman clone of Kasumi possessing highly damaging attacks that can deplete 2/3 of the player's health in one go, as well as strong evasive abilities. Her AI also tends to be extremely punishing in its exploitation of the game mechanics, constantly hitting the player before they're able to reach her, or teleporting away and attacking from behind. If the player instead tries to stay back and defend, she will instantly spring forward and use one of her extremely powerful throws. Her limited moveset means she can be easily countered once players have learned to recognise her combos and she tends to be vulnerable to characters with greater range or good lunging attacks.
In Dead or Alive 5, Alpha-152 returns in story mode as a boss to the False Kasumi and she plays pretty much the same as before. After fighting Ryu Hayabusa, however, she is able to eliminate the major weakness of her predictability by gaining the ability to transform between Kasumi and Ryu, and then later Ayane and Hayate, allowing her to make some damaging combos that are virtually impossible for the player to replicate due to her character switching ability. This also means the player will have to quickly recognize and adapt to which moveset they're fighting on short notice.
In Dead or Alive 6, Raidou once again is a final boss, but only in story mode where either Kasumi, Ayane, or Hayate can fight him. Raidou plays pretty much the same as in normal gameplay, but he constantly has a full meter which means he can perform a break blow at any given time. He also has a special ability to obscure the lighting of the stage which can make it difficult to defend since you're unable to see briefly.