Higher ground is a +10 OB mod. Surprise is +20. Rear is +20. Bonus could realistically be +50 on the first round, and +10/+25/+30 on consecutive rounds depending on flank/rear application.
Also, IF the initial attack hits, I would add to the hit result for rate object is falling/mass, just like the charging rules. This extra OB add would then be used to fugure rap around damage on the table.
Range penalties will probably be quite severe for hitting with a dropped item. Missile weapons are better of course.