Boss death explosion

Hello n my mod I am trying to have it so for the final phase of my boss entity freezes like stops for like about 5 or so seconds maybe with like a glow or pulse then for it to cause an explosion upon its death with soul fire and soul particles explode out of it

I tried do to it but it never actually worked but the only success I've had was with the deaths of its previous phases just cause an explosion and particles but i cant seem to get a prolonged death

any help would be greatly appreciated

the code I used:
https://imgur.com/a/AZErxd2