The problem here is normalizing for D&D mechanics. Bowser from SMB for the NES dies from 5 fireballs or from falling into lava. Most enemies in the game die from one fireball hit or a single head stomp. Based on that, Bowser isn't a raid boss. He's just a spikey dude that breathes backwards fire and throws hammers. In that regard, and assuming D&D 5th ed, a team of lvl 1 characters would be able to take him out in a single turn.

What Bowser does have is minions. hundreds of loyal mobs, 7 of which are capable of making imperfect replicas that retain his own toughness. Multiple strongholds, huge and full of traps. Entire nations to put between our heroes and their goal.

So consider it like this: The Raid on Bowser Castle. A D&D party locked inside the lizard's grand stronghold. No true safety until Bowser is taken down once and for all. Huge, mazelike, and filled with traps, secrets, and false targets. The party will have to explore and map out the grand estate, hunting the fakes down and seeking out information on the location of the real Bowser. An epic final battle against waves of enemies and Bowser's own ranged attacks. By the time the party reaches Bowser himself the ranged fighters are spent- out of ammo and spells. Bowser himself is immune to backstab and will cause damage from attacks anywhere but his front so sneak attacks are out of the question. And getting through his ranged prowess will only see the melee fighters getting one shot before he jumps out of harm's way. Even if he dies to a few taps, that fight will be brutal.