I mostly used the BJ-1 with STD200, AC20, 3MLas, 3t ammo. That got me both of my true beastly rounds. Towards the end, when I ran the numbers and saw that a full assist count was as good as a partial assist count and several kills, I threw together a dakkajack, 2AC2, 4MLas.
Interesting. AC/20 is usually my go-to weapon on the BlackJack, but with all those 100+kph mediums waving around I found myself to slow to follow the pace of the game (I mostly play conquest, maybe that's why). Hence the big engine + LLas build.
I considered a 1 (ER)LLas and x-MLas boat, but didn't have the BJ-3 or -1X past basic and didn't feel like leveling them mid contest, or GXPing for the sake of tourney cheese. So I threw together the Dakkajack.