[Glory of the Hero] / [Lockdown!] BOSS

Hello buddies.
My question is simple. Since no one answered me in global chat I am asking here.
Do you know the spawn rate of boss Lavanthor (2 headed lava dog)?
I am playing VH every day since I don't know when and I've never had it.
It's very upsetting because I need this boss to finish FULL achievement [Glory of the Hero] and I have to wait 24 HR run this again over and over until I get the boss.
So... can you tell me when do you think I will get it? I can't keep doing this my whole life...
Thank you and have a nice day!