Current Black Star Vs Current Laxus Dreyar
-No holding back
-Win by death
-Both serious and bloodlusted
I see Black Star winning by cutting Laxus' head clean off.
Black Star is also physically stronger than Laxus IMO since he has shown better physical feats than Laxus have.
As for Fairy Law... well Black Star wont give Laxus time to complete it cause Black Star can speed blitz him in that moment and cut him in half then lets Tsubaki eat the Soul of Laxus thus becoming a Thunder Dragon Slayer as well and making Black Star and Tsubaki a more badass team then what they are now.