Encounters

[MrDeimos]

I'm having some troubles balancing out combat and players. I have 4 level 12s in my party and yet it seems that they can destroy any creature I throw at them (I threw a CR 15 and a CR 13 at them while they were level 10 and with absolutely no problem they killed them)

I feel something is unbalanced but I'm not quite understanding what I'm doing wrong as a DM