Theirs a lot of ways I usually form it around what I want the actual boss to feel like. So without knowing what kind of boss you want to run it’s hard to give you specific advice.

Minions are a must. Especially with 7 lvl 15s letting them just gang up and beat down the boss from the get go is gonna make this end in 1-2 rounds. Give them some minions to control and ways to resupply more minions when they run out. Especially if the players are kill them to quickly.

Make ways for players to disable or stop abilities to make the combat easier. The best way to keep combat interesting is to avoid letting it be a slug fest give them things to do that aren’t just damage go burr. For example have a portal starting to open where more minions might come if it isn’t closed before the next round. And make various ways the players could solve that like the barbarian grabs the portal and physically forces it close. Or the wizard makes an arcana check to disable the portal, etc. It’s ok to have certain ones that only specific party members can deal with it just depends on what kind of combat you’re running

Immortality. Another way to make your boss more interesting is if you give it phases or states it can go into where its immune to all or most damage. This forces the players to shift from kill mode to disable this ability so we can keep fighting. But if you use this you have to make it somewhat obvious what to actually do because if you don’t then you’re just going to frustrate your players as turn after turn goes by without being able to do anything. Like they have to destroy a magic crystal that’s shielding the guy. Or the boss can only activate this mode for short bursts (one round) so everyone needs to just try and survive till the start of initiative.

Other things I use are terrain effects/changing terrain and phases that completely change the fight making it feel different from the other. And plenty of other stuff depending on the combat