I have an issue that I feel is making my fights boring and repetitive: it's near impossible to not make "focus fire on the boss" the first, second and only choice of strategy for the PCs. I tried to introduce minions, mooks or even elite enemies...and players just ignored them to focus fire on the boss. They once stood inside wall of fire just to keep sniping the boss until he got too far away. I try to get the boss in an interesting enviroment that can impede the PCs, they'll immediatelly do anything to get the boss OUT of that area so they can focus fire. Only time I managed to get them to vary their tactic was when there were two bosses, one far out of reach, and a single PC had personal quest obiective to accomplish during the fight, that also happenned to help the party. But those aren't things I can pull out every boss fight.
Well how about this. Give your boss one or more minions armed with Warding Bond. The fight starts with one of them having cast it on the boss previously- the boss gets +1 AC and +1 saves and takes half damage from everything until that minion dies.
Then the next takes over casting the spell and so on. Oh and of course, the minions have a few healing spells. A level 3 caster is a CR 1/2- heck, you could drop 3rd level casting on a higher CR creature without much incident.
You can also make things harder on them, with dodge and Sanctuary, but that might not teach them the lesson you're wanting them to learn, lol. I have an encounter like this planned for my own game, with this guy:
Koth-Tet, Bound Mummy
Medium Undead, Lawful Evil
AC: 11 (natural armor)
HP: 58 (9d8+18)
Speed: 20 ft.
Str: 18 +4
Dex: 8 -1
Con: 19 +4
Int: 7 -2
Wis: 15 +2
Cha: 12 +1
PB: +3
Saves: Wis +5
Skills: Perception +5
Damage Resistances: bludgeoning, piercing, and slashing
Damage Vulnerabilities: fire, radiant
Damage Immunities: necrotic, poison
Condition Immunities: charmed, exhaustion, frightened, paralyzed, poisoned
Senses: darkvision 60 ft. passive Perception 15
Languages: Celestial, Draconic
CR: 5 (1,800 xp)
Tethered: a spectral chain binds Koth-Tet to Cadweth, keeping him from straying more than 10 ft. away. He cannot be forcibly moved farther than this as long as Cadweth is alive. Further, this tether gives Cadweth the benefits of Warding Bond, and Koth-Tet takes half the damage for his unwanted master. If Cadweth is actually killed, Koth-Tet will aid his unwitting savior, then ask for the mercy of death.
Multiattack: the mummy can use Dreadful Glare and make one attack with it's Rotting Fist.
Rotting Fist: +7 to hit, 11 (2d6) bludgeoning damage plus 14 (4d6) necrotic damage. The target must make a DC 15 Con save or be cursed with mummy rot. The cursed target cannot regain hit points and their hit point maximum decreases by 14 (4d6) for every 24 hours that elapse. If the curse reduces your maximum to 0, the target dies and they turn to dust. The curse lasts until removed by remove curse or other magic.
Dreadful Glare: targets one creature the mummy can see within 60 feet. The target must make a DC 13 Wis save or be frightened until the end of the mummy's next turn. If the target fails this save by 5 or more, they are also paralyzed for the same duration. Once you succeed on this save you become immune the glare for 24 hours.