Hello,
It's possible to do something like that in LUA, but you'll find some problems. Here you have all the function you need to make something like you want:
local mob_1 = d.spawn_mob(VNUM, X, Y)
d.set_unique("mob_1_unique", mob_1)
d.unique_set_maxhp("mob_1_unique", HP)
d.unique_set_hp("mob_1_unique", HP)
d.unique_get_hp_perc("mob_1_unique")
d.unique_set_def_grade("mob_1_unique", 99999)
All the functions are self-explanatory, you just need to use some triggers to call them (like server_timer, kill, login, etc).
There isn't a way (with the actual functions) to block the hp from the mob you want, but you can use the last function to increase it's defense. Doing this, you'll find one problem: piercing damage will take affect on the defense given to the mob, so this isn't a good method.
Hope I helped you in some way. If you have any question, you can send me a message on my discord: Doka#5676
Sincerely,
Esculpe