could have a table in the boss called resettimer or something like that, so when boss hp = 0 it will insert a random number into that from say 120 to 240 (represents mins) then add a cron min to count this down
in header, something like if resettimer= < 60 then "new boss in mins left"
in the cron check for 5 mins left then re fill the bosses health,
Pay crystals for an attack boost on the boss (each attack you land does 5% more damage stacking upto 10 times 50% total) (this buff lasts for the duration that the boss is alive)
if the boss doesn't die after a set period of time, anyone that attacked it will be hospitalized for x ammount of time,