I didn't get to use Battlesystem on Saturday as the party hasn't finished clearing out the main cavern of the Vile Rune orc tribe. Although they did probably broke their back when they slaughtered the king and his court while they were feasting and betting on a fight between a panther and a bear.
During prep I noticed I needed to handle Table 9: Ration Adjustments for Determining AR in section 8.2
I did run a short playtest to see how it would out. I have to say the fighter types against 1 HD creature just rule as individuals. I manage to have one guy to 50 HD of damage in one Battle System round wiping out five figures of orcs (50 orcs). I know in some editions that only works if they are HD 1-1 or less. In my S&W/Majestic Wilderlands I allow it for HD 1 or less due to the lack of multiple attacks at higher levels.
After playing around with the number I found that I need to subtract the above to use this with a To Hit Bonus and Ascending AC. So Sir Mighty a fighter who has +8 to hit will have an AR of -7 in Ascending AC Battlesystem. A group of 4 HD Ogres in Swords & Wizardry have an AR of -1 due to the fact they are setup as figures with a 5:1 to ration. The ogres are +4 to hit - 5 for the 5:1 ration = AR -1.
I also did some looking at the DCC RPG and I feel it would work as far as spells goes. It would be a more random compared to D&D due to the spell charts. But however it would probably be more interesting due to the increased variety of results. I will look at the fighters next and see how that would work out.